As filed with the Securities and Exchange Commission on August 24, 2023.

Registration No. 333-274068

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Approximate date of commencement of proposed sale to the public: From time to time after the effective date of this registration statement.

If the only securities being registered on this Form are being offered pursuant to dividend or interest reinvestment plans, please check the following box.o

If any of the securities being registered on this Form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of 1933, other than securities offered only in connection with dividend or interest reinvestment plans, check the following box.x

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, please check the
following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.o

If this Form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.o

If this Form is a registration statement pursuant to General Instruction I.D. or a post-effective amendment thereto that shall become effective upon filing with the Commission pursuant to Rule 462(e) under the Securities Act, check the following box.o

If this Form is a post-effective amendment to a registration statement filed pursuant to General Instruction I.D. filed to register additional securities or additional classes of securities pursuant to Rule 413(b) under the Securities Act, check the following box.o

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company. See the definitions of “large accelerated filer,” “accelerated filer,” “smaller reporting company” and “emerging growth company” in Rule 12b-2 of the Exchange Act.

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If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of the Securities Act. o

The registrant hereby amends this registration statement on such date or dates as may be necessary to delay its effective date until the registrant shall file a further amendment which specifically states that this registration statement shall thereafter become effective in accordance with Section 8(a) of the Securities Act of 1933 or until this registration statement shall become effective on such date as the Securities and Exchange Commission, acting pursuant to said Section 8(a), may determine.

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The information in this prospectus is not complete and may be changed. This prospectus is included in a registration statement that we filed with the Securities and Exchange Commission. We may not sell these securities until the registration statement filed with the Securities and Exchange Commission is effective. This prospectus is not an offer to sell these securities and it is not soliciting an offer to buy these securities in any jurisdiction where the offer or sale is not permitted.

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Subject to completion, dated August 24, 2023

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In this prospectus, unless the context suggests otherwise, references to the “Company,” “we,” “us,” and “our” mean the combined business of Old Second Bancorp, Inc. and its subsidiary bank, Old Second National Bank (the “Bank”).

We may offer and sell, at any time and from time to time, in one or more offerings, and in any combination, the securities described in this prospectus having a total initial offering price not exceeding $200,000,000. This prospectus describes the general terms of these securities and the general manner in which we will offer them. Each time that we offer and sell securities using this prospectus, we will provide a supplement to this prospectus that contains specific information about the securities and their terms and the manner in which we will offer them for sale. The prospectus supplement also may add or update information contained in this prospectus. You should carefully read this prospectus and any prospectus supplement, as well as any documents we have incorporated into this prospectus or any prospectus supplement by reference, before you invest in any of these securities. This prospectus may not be used to consummate the sales of securities unless accompanied by a prospectus supplement.

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We may offer and sell these securities to or through one or more underwriters, dealers or agents, or directly to purchasers on a continuous or delayed basis. We will provide the names of any such underwriters, dealers or agents used in connection with the sale of any of these securities, as well as any fees, commissions or discounts we may pay to such underwriters, dealers or agents in connection with the sale of these securities, in the applicable prospectus supplement.

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Our principal executive office is located at 37 South River Street, Aurora, Illinois 60507 and our telephone number is (630) 892-0202. Our common stock is listed on the NASDAQ Global Select Market under the symbol “OSBC.” On August 23, 2023, the last reported sale price of our common stock was $14.40. We have not yet determined whether any of the other securities that may be offered pursuant to this prospectus will be listed on any exchange. If we decide to do so, a prospectus supplement relating to such securities will identify the exchange or market on which they will be listed.

These securities are not bank deposits and are not insured by the Federal Deposit Insurance Corporation or any other governmental agency, nor are they obligations of, or guaranteed by, a bank.

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Investing in our securities involves risks. You should refer to the section entitled “Risk Factors” on page 6 of this prospectus, as well as the risk factors included in the applicable prospectus supplement and in the reports we file with the Securities and Exchange Commission that are incorporated by reference into this prospectus, and carefully consider that information before deciding to invest in any of our securities.

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Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if this prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

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This prospectus is dated August 24, 2023.

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TABLE OF CONTENTS

Prospectus

About this Prospectus‌1

Special Note Regarding Forward-Looking Statements‌2

PROSPECTUS SUMMARY‌4

Risk Factors‌6

Use of Proceeds‌6

Description of Securities We May Offer‌6

Description of Capital Stock‌7

Description of Debt Securities‌11

Description of Warrants‌19

Description of Depositary Shares‌20

Description of Subscription Rights‌22

Description of Stock Purchase Contracts and Stock Purchase Units‌23

Description of Units‌24

Plan of Distribution‌24

Legal Matters‌26

Experts‌26

Where You Can Find More Information‌26

Incorporation of Certain Information By Reference‌26

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About this Prospectus

This prospectus is part of a registration statement that we filed with the Securities and Exchange Commission (the “SEC”) using a “shelf” registration process. Under this shelf registration process, we may sell from time to time, in one or more offerings, on a continuous or delayed basis, any combination of the securities described in this prospectus.

This prospectus only provides you with a general description of the securities we may offer. Each time we offer and sell securities, we will provide a supplement to this prospectus (which may be a pricing supplement, index supplement, product supplement, free writing prospectus and/or other type of offering document or prospectus supplement, and which are together referred to herein as a “prospectus supplement” or a “supplement”) that will contain specific information about the terms of the securities and the offering. A prospectus supplement may include a discussion of any risk factors or other special considerations applicable to those securities or to us. Such prospectus supplement may also add, update or change information contained or incorporated by reference in this prospectus. Any information contained in this prospectus will be deemed to be modified or superseded by any inconsistent information contained in a prospectus supplement. Accordingly, if there is any inconsistency between the information in this prospectus and the applicable prospectus supplement, you should rely on the information in the prospectus supplement. You should read this prospectus and the applicable prospectus supplement and any documents that we incorporate by reference in this prospectus or any prospectus supplement, together with the additional information referred to under the heading “Where You Can Find More Information,” before you invest.

You should rely only on the information contained or incorporated by reference in this prospectus or in any prospectus supplement. We have not authorized anyone to provide you with different or additional information. We are not making an offer to sell or soliciting an offer to buy these securities in any jurisdiction in which the offer or solicitation is not authorized or in which the person making the offer or solicitation is not qualified to do so or to anyone to whom it is unlawful to make the offer or solicitation. You should assume that the information contained or incorporated by reference in this prospectus or any prospectus supplement is accurate only as of the date of that document.

Any of the securities described in this prospectus and in a prospectus supplement may be convertible or exchangeable into, or exercisable for, other securities that are described in this prospectus or will be described in a prospectus supplement, and may be issued separately, together or as part of a unit consisting of two or more securities, which may or may not be separate from one another. The securities offered hereby may include new or hybrid securities developed in the future that combine features of any of the securities described in this prospectus.

The registration statement that contains this prospectus, including the exhibits to the registration statement, also contains additional information about us and the securities offered under this prospectus. You can find the registration statement at the SEC’s website at www.sec.gov.

Unless otherwise indicated, currency amounts in this prospectus and in any applicable prospectus supplement are stated in United States dollars.

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Special Note Regarding Forward-Looking Statements

This prospectus, any related prospectus supplement, and the documents we incorporate by reference in this prospectus and any related prospectus supplement may contain forward-looking statements within the meaning of the federal securities laws, including with respect to management’s current beliefs and expectations regarding future plans, strategies and financial performance, regulatory developments, industry and economic trends, and other matters.  Forward-looking statements are identifiable by the inclusion of such qualifications as “expects,” “intends,” “believes,” “may,” “will,” “would,” “could,” “should,” “plan,” “anticipate,” “estimate,” “possible,” “likely” or other indications that the particular statements are not historical facts.  Actual events and results may differ significantly from those described in such forward-looking statements, due to numerous factors, including:

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These and other risks and uncertainties are more fully discussed in the “Risk Factors” and other sections of the documents that we incorporate by reference into this prospectus, including our Annual Reports on Form 10-K and our Quarterly Reports on Form 10-Q and in our other reports filed with the SEC. Should one or more of these risks or uncertainties materialize, or should underlying assumptions prove incorrect, actual outcomes may vary materially from those anticipated. All subsequent written and oral forward-looking statements attributable to us or persons acting on our behalf are expressly qualified in their entirety by reference to these risks and uncertainties. We undertake no obligation to update any forward-looking statement to reflect factual assumptions, circumstances or events that have changed after we have made the forward-looking statements, except as required by law. You should not put undue reliance on any forward-looking statements.

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PROSPECTUS SUMMARY

This summary provides a general description of the securities we may offer. This summary is not complete and does not contain all of the information that may be important to you. For a more complete understanding of us and the terms of the securities we may offer, you should read carefully this entire prospectus, including the “Risk Factors” section, the applicable prospectus supplement for the securities and the other documents we refer to and incorporate by reference. In particular, we incorporate important business and financial information into this prospectus by reference.

Old Second Bancorp, Inc.

Old Second Bancorp, Inc. is a corporation organized under the laws of the State of Delaware in 1981 that serves as the bank holding company for its wholly-owned subsidiary bank, Old Second National Bank. The Bank is a national banking association headquartered in Aurora, Illinois, that operates through 48 banking centers located in Cook, DeKalb, DuPage, Kane, Kendall, LaSalle and Will counties in Illinois.

We are a full-service banking business offering a broad range of deposit products, trust and wealth management services, and lending services, including demand, NOW, money market, savings, time deposit and individual retirement accounts; commercial, industrial, consumer and real estate lending, including installment loans, agricultural loans, lines of credit and overdraft checking; safe deposit operations, and an extensive variety of additional services tailored to the needs of individual customers, such as the acquisition of U.S. Treasury notes and bonds, money orders, cashiers’ checks and foreign currency, direct deposit, discount brokerage, debit cards, credit cards, and other special services. Our lending activities include making commercial and consumer loans, primarily on a secured basis. Commercial lending focuses on business, capital, construction, inventory and real estate lending. Installment lending includes direct and indirect loans to consumers and commercial customers.

We also offer a full complement of electronic banking services such as online and mobile banking and corporate cash management products including remote deposit capture, mobile deposit capture, investment sweep accounts, zero balance accounts, automated tax payments, ATM access, telephone banking, lockbox accounts, automated clearing house transactions, account reconciliation, controlled disbursement, detail and general information reporting, foreign and domestic wire transfers, vault services for currency and coin, and checking accounts. Additionally, we provide a wide range of wealth management, investment, agency, and custodial services for individual, corporate, and not-for-profit clients. These services include the administration of estates and personal trusts, as well as the management of investment accounts for individuals, employee benefit plans, and charitable foundations. We also originate residential mortgages, offering a wide range of mortgage products including conventional, government, and jumbo loans. We also handle secondary marketing of those mortgages.

As of June 30, 2023, we had consolidated total assets of $5.88 billion, net loans of $3.96 billion, deposits of $4.72 billion and stockholders’ equity of $514.0 million.

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Our principal executive office is located at 37 South River Street, Aurora, Illinois 60507 and our telephone number is (630) 892-0202.

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Additional information about us is included in our filings with the SEC, which are incorporated by reference into this prospectus. See “Where You Can Find More Information” and “Incorporation of Certain Information by Reference” in this prospectus.

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The Securities We May Offer

Under the shelf registration statement to which this prospectus is a part, we may sell securities consisting of one or any combination or combinations of the following securities or any combination of these securities from time to time:

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A prospectus supplement, which we will provide each time we offer securities, will describe the amounts, prices and detailed terms of the securities and may describe risks associated with an investment in the securities in addition to those described in the “Risk Factors” section on page 6 of this prospectus.

We may sell the securities to or through underwriters, dealers or agents or directly to purchasers. Each prospectus supplement will set forth the names of any underwriters, dealers or agents involved in the sale of such securities described in that prospectus supplement and any applicable fee, commission or discount arrangements with them.

Listing

If any securities are to be listed or quoted on a securities exchange or quotation system, the applicable prospectus supplement will so indicate. Our common stock is listed on the NASDAQ Global Select Market under the symbol “OSBC.”

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Risk Factors

An investment in our securities involves certain risks. Before you invest in any of our securities, in addition to the other information included in, or incorporated by reference into, this prospectus, you should carefully read and consider the risk factors contained in our most recent Annual Report on Form 10-K, and in our updates to those Risk Factors in our Quarterly Reports on Form 10-Q and Current Reports on Form 8-K, which are incorporated by reference into this prospectus. See “Incorporation of Certain Information by Reference” and “Where You Can Find More Information.” Our business, financial condition or results of operations could be materially adversely affected by any of these risks. The trading price of our securities could decline due to any of these risks, and you may lose all or part of your investment. This prospectus and the documents incorporated by reference into this prospectus also contain forward-looking statements that involve risks and uncertainties. Our actual results could differ materially from those anticipated in these forward-looking statements as a result of certain factors, including the risks faced by us.

Use of Proceeds

Unless the applicable prospectus supplement states otherwise, we will use the net proceeds we receive from the sale of the securities offered hereby for general corporate purposes, which may include, among other things, investments in or advances to our subsidiaries, working capital, capital expenditures, stock repurchases, debt repayment or the financing of possible acquisitions. The applicable prospectus supplement relating to a particular offering of securities by us will identify the particular use of proceeds for that offering. Until we use the net proceeds from an offering, we may place the net proceeds in temporary investments or hold the net proceeds in deposit accounts at the Bank.

Description of Securities We May Offer

This prospectus contains summary descriptions of the common stock, preferred stock, debt securities, warrants, depositary shares, subscription rights, stock purchase contracts, stock purchase units and units that we may offer and sell from time to time. When one or more of these securities are offered in the future, a prospectus supplement will explain the particular terms of the securities and the extent to which these general provisions may apply. These summary descriptions and any summary descriptions in the applicable prospectus supplement do not purport to be complete descriptions of the terms and conditions of each security and are qualified in their entirety by reference to our Restated Certificate of Incorporation, as amended (our “Certificate of Incorporation”), our Bylaws, as amended (our “Bylaws”), the Delaware General Corporation Law, as amended (the “DGCL”) and any other documents referenced in such summary descriptions and from which such summary descriptions are derived. If any particular terms of a security described in the applicable prospectus supplement differ from any of the terms described in this prospectus, then the terms described in this prospectus will be deemed superseded by the terms set forth in that prospectus supplement.

We may issue securities in book-entry form through one or more depositaries, such as The Depository Trust Company, Euroclear or Clearstream, named in the applicable prospectus supplement. Each sale of a security in book-entry form will settle in immediately available funds through the applicable depositary, unless otherwise stated. We will issue the securities in registered form, without coupons, although we may issue the securities in bearer form if so specified in the applicable prospectus supplement. If any securities are to be listed or quoted on a securities exchange or quotation system, the applicable prospectus supplement will say so.

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Description of Capital Stock

The following description of our capital stock is a summary only and is subject to, and is qualified in its entirety by reference to, applicable provisions of the DGCL, our Certificate of Incorporation and our Bylaws. Although we believe this summary covers the material terms and provisions of our capital stock set forth in our Certificate of Incorporation and Bylaws, it may not contain all of the information that is important to you.

In this section “Description of Capital Stock,” the “Company,” “we,” “our,” or “us” refer only to Old Second Bancorp, Inc. and not to any of its subsidiaries.

Authorized Shares of Capital Stock

We have the authority to issue 60,000,000 shares of common stock, $1.00 par value per share, and 300,000 shares of preferred stock, $1.00 par value per share.

Common Stock

General. As of the date of this prospectus, our Certificate of Incorporation provides that we may issue up to 60,000,000 shares of common stock, $1.00 par value per share. As of June 30, 2023, we had 44,705,150 shares of common stock issued and 44,665,127 shares of our common stock outstanding. There were 764,606 shares of our common stock underlying restricted stock unit awards pursuant to our equity incentive plans and 967,600 shares of our common stock remaining available for future grant under the Old Second Bancorp, Inc. 2019 Equity Incentive Plan, as amended and restated. All outstanding shares of our common stock are fully paid and nonassessable.

Our common stock is listed on the NASDAQ Global Select Market under the symbol “OSBC.”

Dividend Rights. Subject to any rights to receive dividends to which the holders of any outstanding preferred stock may be entitled, holders of our common stock are entitled to receive any cash dividends that may be declared by our board of directors. We are subject to certain restrictions on dividend payments under the DGCL as well as restrictions under certain banking laws, regulations and policies. Subject to these restrictions, the declaration and payment of future dividends to holders of our common stock will be at the discretion of our board of directors and will depend upon our earnings and financial condition, our capital requirements and those of our subsidiaries, regulatory conditions and considerations and other factors as our board of directors may deem relevant.

Voting Rights. Each outstanding share of our common stock shall entitle the holder thereof to one vote, and, except as otherwise stated or expressed in a resolution or resolutions adopted by our board of directors providing for the issue of any preferred stock or as otherwise provided by law, the exclusive voting power for all purposes shall be vested in the holders of common stock. The holders of our common stock have no cumulative voting rights. When a quorum is present at any meeting, the vote of a majority of the shares present in person or represented by proxy will decide the election of directors, unless the number of director nominees exceeds the number of directors to be elected at such meeting, in which case, the directors will be elected by a plurality vote. When a quorum is present at any meeting, questions brought before the meeting will be decided by the vote of the holders of a majority of the shares having voting power present in person or represented by proxy, except when the matter requires a different vote under applicable Delaware law, our Certificate of Incorporation or our Bylaws. Our Certificate of Incorporation provides certain provisions that may limit stockholders’ ability to effect a change in control as described under the section below entitled “—Antitakeover Provisions—Certificate of Incorporation; Bylaws.”

Liquidation Rights. In the event of any liquidation, dissolution or winding up of the Company, whether voluntary or involuntary, after payment or provision for payment of the debts and other liabilities of the Company and the preferential amounts to which the holders of any outstanding preferred stock shall be entitled, the holders of our common stock shall be entitled to share ratably in the remaining assets of the Company. The merger or consolidation of the Company into or with any other corporation, or the merger of any other corporation into it, or a sale of all or substantially all of the assets of the Company, or, any purchase or redemption of shares of stock of the Company of any class, shall not be deemed to be a liquidation, dissolution or winding up of the Company.

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Preemptive Rights. The holders of our common stock have no preemptive rights.

Miscellaneous. Shares of our common stock are not convertible into shares of any other class of capital stock.

Preferred Stock

General. We may issue up to 300,000 shares of preferred stock, $1.00 par value per share, from time to time in one or more series. Our board of directors, without further approval of our stockholders, has the authority to fix the dividend rights and terms, conversion rights, voting rights, redemption rights and terms, liquidation preferences, sinking funds and any other rights, preferences, privileges and restrictions applicable to each series of preferred stock. The issuance of preferred stock, while providing flexibility in connection with possible acquisitions and other corporate purposes, could, among other things, adversely affect the voting power of the holders of our common stock.

The authorization of preferred stock could have the effect of impeding the acquisition of control of the Company by means of a tender offer, a proxy fight, open-market purchases or otherwise in a transaction not approved by our board of directors. See “—Antitakeover Provisions—Certificate of Incorporation; Bylaws.”

As of June 30, 2023, we had no shares of preferred stock issued and outstanding.

We will describe the particular terms of any series of preferred stock being offered in the applicable prospectus supplement relating to that series of preferred stock. The following description and any description of preferred stock in a prospectus supplement is only a summary and is subject to and qualified in its entirety by reference to the certificate of amendment to our Certificate of Incorporation relating to the particular series of preferred stock, a copy of which we will file with the SEC in connection with the sale of any series of preferred stock.

The prospectus supplement relating to a particular series of preferred stock will contain a description of the specific terms of that series of preferred stock. Those terms may include:

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The shares of preferred stock will, when issued against full payment of their purchase price, be fully paid and nonassessable. Except as otherwise may be specified in the prospectus supplement relating to a particular series of preferred stock, holders of preferred stock will not have any preemptive or subscription rights to acquire any class or series of our capital stock.

Dividend Rights. If you purchase preferred stock being offered by use of this prospectus and an applicable prospectus supplement, you will be entitled to receive, when, as and if declared by our board of directors, dividends at the rates and on the dates set forth in the prospectus supplement. Dividend rates may be fixed, variable or both. The nature, amount, rates, timing and other details of dividend rights for a series of preferred stock will be described in the applicable prospectus supplement and will be payable in preference to, or in such relation to, the dividends payable on any other class or classes or series of our stock, as described in the applicable prospectus supplement. We are subject to various regulatory policies and requirements relating to the payment of dividends, including requirements to maintain adequate capital above regulatory minimums.

Voting Rights. The voting rights of preferred stock of any series being offered will be described in the applicable prospectus supplement.

Liquidation Rights. In the event that we liquidate, dissolve or wind-up our affairs, either voluntarily or involuntarily, holders of our preferred stock will be entitled to receive liquidating distributions in the amount set forth in the applicable prospectus supplement, plus accrued and unpaid dividends, if any, before we make any distribution of assets to the holders of our common stock or any junior preferred stock. If we fail to pay in full all amounts payable with respect to preferred stock being offered by us and any stock having the same rank as that series of preferred stock, the holders of the preferred stock and of that other stock will share in any distribution of assets in proportion to the full respective preferential amounts to which they are entitled. After the holders of each series of preferred stock and any stock having the same rank as the preferred stock are paid in full, they will have no right or claim to any of our remaining assets. For any series of preferred stock being offered by this prospectus and an applicable prospectus supplement, neither the sale of all or substantially all of our property or business nor a merger or consolidation by us with any other corporation will be considered a dissolution, liquidation or winding-up of our business or affairs.

Redemption. The terms, if any, on which shares of a series of preferred stock being offered may be redeemed will be described in the applicable prospectus supplement. The preferred stock of a series may be redeemed in such amount or amounts, and at such time or times, if any, as may be provided in respect of that particular series of preferred stock. Preferred stock may be redeemed by the Company only to the extent legally permissible.

Conversion Rights. The applicable prospectus supplement will state the terms, if any, on which shares of a series of preferred stock being offered are convertible into shares of our common stock or another series of our preferred stock.

Anti-Takeover Provisions

General. Certain provisions of our Certificate of Incorporation, our Bylaws and the DGCL may have the effect of impeding the acquisition of control of the Company by means of a tender offer, a proxy fight, open-market purchases or otherwise in a transaction not approved by our board of directors. These provisions may have the effect of discouraging a future takeover attempt which is not approved by our board of directors, but which individual stockholders may deem to be in their best interests or in which our stockholders may receive a substantial premium for their shares over then-current market prices. As a result, stockholders who might desire to participate in such a transaction may not have an opportunity to do so. Such provisions will also render the removal of our current board of directors or management more difficult.

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Certificate of Incorporation; Bylaws. These provisions of our Certificate of Incorporation and our Bylaws include the following:

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Delaware Law. We have not elected to opt out of the applicability of Section 203 of the DGCL in our Certificate of Incorporation. Under Section 203 of the DGCL, subject to exceptions, we are prohibited from engaging in any business combination with any interested stockholder for a period of three years following the time that the stockholder became an interested stockholder. For this purpose, an “interested stockholder” generally includes current and certain former holders of 15% or more of our outstanding stock. The provisions of Section 203 may encourage companies interested in acquiring us to negotiate in advance with our board of directors. These provisions may make it more difficult to accomplish transactions which stockholders may otherwise deem to be in their best interests.

Banking Laws. The ability of a third party to acquire the Company is also subject to applicable banking laws and regulations. The Bank Holding Company Act of 1956 and the regulations thereunder require any “bank holding company” (as defined therein) to obtain the approval of the Federal Reserve prior to acquiring more than 5% of the outstanding shares of a class of our voting stock. Any person other than a bank holding company is required to obtain prior approval of the Federal Reserve to acquire 10% or more of the outstanding shares of a class of our voting stock under the Change in Bank Control Act of 1978. Federal law also prohibits any person or company from acquiring “control” of an FDIC-insured depository institution or its holding company without prior notice to the appropriate federal bank regulator. On January 30, 2020, the Federal Reserve issued a final rule (which became effective September 30, 2020) that clarified and codified the Federal Reserve’s standards for determining whether one company has control over another. The final rule established four categories of tiered presumptions of control that are based on the percentage of voting shares held by the investor (less than 5%, 5-9.9%, 10-14.9% and 15-24.9%) and the presence of other indicia of control. As the percentage of ownership increases, fewer indicia of control are permitted without falling outside the presumption of noncontrol. These indicia of control include nonvoting equity ownership, director representation, management interlocks, business relationship and restrictive contractual covenants. Under the final rule, investors can hold up to 24.9% of the voting securities and up to 33% of the total equity of a company without necessarily having a controlling influence. For purposes of calculating ownership thresholds under these banking regulations, bank regulators would likely at least take the position that the minimum number of shares, and could take the position that the maximum number of shares, of a company’s common stock that a holder is entitled to receive pursuant to securities convertible into or settled in such company’s common stock, including pursuant to warrants to purchase such company’s common stock held by such holder, must be taken into account in calculating a stockholder’s aggregate holdings of such company’s common stock.

Description of Debt Securities

In this section “Description of Debt Securities,” the “Company,” “we,” “our,” or “us” refer only to Old Second Bancorp, Inc. and not to any of its subsidiaries.

General

The debt securities that we may offer using this prospectus consist of notes, debentures or other evidences of indebtedness. Any debt securities that we offer and sell will be our direct obligations. Debt securities may be issued in one or more series. All debt securities of any one series need not be issued at the same time, and unless otherwise provided, a series of debt securities may be reopened, without the consent of the holders of outstanding debt securities, for issuance of additional debt securities of that series or to establish additional terms of that series of debt securities (with such additional terms applicable only to unissued or additional debt securities of that series).

As required by the Trust Indenture Act of 1939, as amended (the “Trust Indenture Act”), for all debt securities that are publicly offered, our debt securities will be governed by a document called an indenture to be entered into by and between us and such bank or trust company that we select to act as trustee (the “trustee”). The form of indenture is subject to any amendments or supplements that we may enter into with the trustee(s). The indenture is filed as an exhibit to the registration statement of which this prospectus forms a part. The following description and any description in a prospectus supplement is a summary only and is subject to, and qualified in its entirety by reference to the terms and provisions of the indentures and any supplemental indentures that we file with the SEC in connection with an issuance of any series of debt securities. You should read all of the provisions of the indentures, including the definitions of certain terms, as well as any supplemental indentures that we file with the SEC in connection with the issuance of any series of debt securities. These summaries set forth certain general terms

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and provisions of the securities to which any prospectus supplement may relate. The specific terms and provisions of a series of debt securities and the extent to which the general terms and provisions may also apply to a particular series of debt securities will be described in the applicable prospectus supplement.

Each prospectus supplement will specify the particular terms of the securities offered. These provisions may include, among other things and to the extent applicable, the following:

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Unless otherwise indicated in the applicable prospectus supplement, we will issue debt securities in fully registered form without coupons and in denominations of $1,000 and in integral multiples of $1,000, and interest will be computed on the basis of a 360-day year of twelve 30-day months. If any interest payment date or the maturity date falls on a day that is not a business day, then the payment will be made on the next business day without additional interest and with the same effect as if it were made on the originally scheduled date.

Unless otherwise indicated in the applicable prospectus supplement, the trustee will act as paying agent and registrar for the debt securities under the indenture. We may also act as paying agent under the indenture.

The applicable prospectus supplement will contain a description of U.S. federal income tax consequences relating to the debt securities, to the extent applicable.

Covenants

The applicable prospectus supplement will describe any covenants, such as restrictive covenants restricting us or any of our subsidiaries from incurring, issuing, assuming or guarantying any indebtedness or restricting us or any of our subsidiaries from paying dividends or acquiring any of our or its capital stock.

Consolidation, Merger and Transfer of Assets

Unless we indicate otherwise in the applicable prospectus supplement, the indenture will permit a consolidation or merger between us and another entity and/or the sale, conveyance or lease by us of all or substantially all of our property and assets; provided, however, that:

If we consolidate or merge with or into any other entity, or sell or lease all or substantially all of our assets in compliance with the terms and conditions of the indenture, the resulting or acquiring entity will be substituted for us in the indenture and the debt securities with the same effect as if it had been an original party to the indenture and the debt securities. As a result, such successor entity may exercise our rights and powers under the indenture and the debt securities, in our name, and, except in the case of a lease, we will be released from all our liabilities and obligations under the indenture and under the debt securities.

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Notwithstanding the foregoing, we may transfer all of our property and assets to another entity if, immediately after giving effect to the transfer, such entity is our wholly-owned subsidiary. The term “wholly-owned subsidiary” means any subsidiary in which we and/or our other wholly-owned subsidiaries own all of the outstanding capital stock.

Modification and Waiver

Unless we indicate otherwise in the applicable prospectus supplement, under the indenture, some of our rights and obligations and some of the rights of the holders of the debt securities may be modified or amended with the consent of the holders of not less than a majority in aggregate principal amount of the outstanding debt securities affected by the modification or amendment. However, the following modifications and amendments will not be effective against any holder without its consent:

Under the indenture, the holders of not less than a majority in aggregate principal amount of the outstanding debt securities may, on behalf of all holders of the debt securities:

Events of Default

Unless we indicate otherwise in the applicable prospectus supplement, “event of default” under the indenture will mean, with respect to any series of debt securities, any of the following:

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Unless we indicate otherwise in the applicable prospectus supplement, if an event of default occurs and continues, the trustee or the holders of not less than 25% in aggregate principal amount of the outstanding debt securities of such series may declare the entire principal of all the debt securities to be due and payable immediately, except that, if the event of default described in the fourth bullet point above occurs, the entire principal of all of the debt securities of such series will become due and payable immediately without any act on the part of the trustee or holders of the debt securities. If such a declaration occurs, the holders of a majority of the aggregate principal amount of the outstanding debt securities of such series can, subject to conditions, rescind the declaration.

The indenture requires us to furnish to the trustee, not less often than annually, a certificate from our principal executive officer, principal financial officer or principal accounting officer, as the case may be, as to such officer’s knowledge of our compliance with all conditions and covenants under the indenture. The trustee may withhold notice to the holders of debt securities of any default, except defaults in the payment of principal of, or interest on, any debt securities if the trustee in good faith determines that the withholding of notice is in the interests of the holders. For purposes of this paragraph, “default” means any event which is, or after notice or lapse of time or both would become, an event of default under the indenture.

The trustee is not obligated to exercise any of its rights or powers under the indenture at the request, order or direction of any holders of debt securities, unless the holders offer the trustee satisfactory security or indemnity. If satisfactory security or indemnity is provided, then, subject to other rights of the trustee, the holders of a majority in aggregate principal amount of the outstanding debt securities may direct the time, method and place of:

The holder of a debt security will have the right to begin any proceeding with respect to the indenture or for any remedy only if:

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However, the holder of any debt security will have an absolute right to receive payment of principal of, and interest on, the debt security when due and to institute suit to enforce payment.

Satisfaction and Discharge; Defeasance

Satisfaction and Discharge of Indenture. Unless otherwise indicated in the applicable prospectus supplement, if at any time,

then the indenture shall cease to be of further effect with respect to the debt securities of such series, except for (a) rights of registration of transfer and exchange, and our right of optional redemption, (b) substitution of mutilated, defaced, destroyed, lost or stolen debt securities, (c) rights of holders to receive payments of principal thereof and interest thereon upon the original stated due dates therefor (but not upon acceleration) and remaining rights of the holders to receive mandatory sinking fund payments, if any, (d) the rights, powers, trusts, duties and immunities of the trustee under the indenture and our obligations in connection therewith, and (e) the rights of the holders of such series of debt securities as beneficiaries thereof with respect to the property so deposited with the trustee payable to all or any of them.

Defeasance and Covenant Defeasance. Unless otherwise indicated in the applicable prospectus supplement, we may elect with respect to any debt securities of any series either:

We must comply with the following conditions before the defeasance or covenant defeasance can be effected:

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In connection with defeasance, any irrevocable trust agreement contemplated by the indenture must include, among other things, provision for (a) payment of the principal of and interest on such debt securities, if any, appertaining thereto when due (by redemption, sinking fund payments or otherwise), (b) the payment of the expenses of the trustee incurred or to be incurred in connection with carrying out such trust provisions, (c) rights of registration, transfer, substitution and exchange of such debt securities in accordance with the terms stated in the indenture, and (d) the rights, powers, trusts, duties and immunities of the trustee under the indenture and our obligations in connection therewith as stated in the indenture.

The accompanying prospectus supplement may further describe any provisions permitting or restricting defeasance or covenant defeasance with respect to the debt securities of a particular series.

Global Securities

Unless otherwise indicated in the applicable prospectus supplement, each debt security offered by this prospectus will be issued in the form of one or more global debt securities representing all or part of that series of debt securities. This means that we will not issue certificates for that series of debt securities to the holders. Instead, a global debt security representing that series will be deposited with, or on behalf of, a securities depositary and registered in the name of the depositary or a nominee of the depositary. Any such depositary must be a clearing agency registered under the Securities Exchange Act of 1934, as amended, or the Exchange Act. We will describe the specific terms of the depositary arrangement with respect to a series of debt securities to be represented by a global security in the applicable prospectus supplement.

Notices

We will give notices to holders of the debt securities by mail at the addresses listed in the security register or, with respect to global securities, in accordance with the rules, policies and procedures of the applicable securities depositary.

Governing Law

The indenture and the debt securities will be governed by, and construed in accordance with, the laws of the State of New York, except to the extent the Trust Indenture Act is applicable.

Regarding the Trustee

General. From time to time, we may maintain deposit accounts and conduct other banking transactions with the trustee to be appointed under the indenture or its affiliates in the ordinary course of business.

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Resignation or Removal of Trustee. If the trustee has or acquires a conflicting interest within the meaning of the Trust Indenture Act, the trustee must either eliminate its conflicting interest or resign, to the extent and in the manner provided by, and subject to the provisions of, the Trust Indenture Act and the indenture. Any resignation will require the appointment of a successor trustee under the indenture in accordance with the terms and conditions of the indenture.

The trustee may resign or be removed by us with respect to one or more series of debt securities and a successor trustee may be appointed to act with respect to any such series. The holders of a majority in aggregate principal amount of the debt securities of any series may remove the trustee with respect to the debt securities of such series.

Annual Trustee Report to Holders of Debt Securities. The trustee will be required to submit certain reports to the holders of the debt securities regarding, among other things, the trustee’s eligibility to serve as such, the priority of the trustee’s claims regarding advances made by it, and any action taken by the trustee materially affecting the debt securities.

Certificates and Opinions to Be Furnished to Trustee. The indenture provides that, in addition to other certificates or opinions specifically required by other provisions of the indenture, every application by us for action by the trustee must be accompanied by a certificate from one or more of our officers and an opinion of counsel (who may be our counsel) stating that, in the opinion of the signers, all conditions precedent to such action have been complied with by us.

Description of Warrants

In this section “Description of Warrants,” the “Company,” “we,” “our,” or “us” refer only to Old Second Bancorp, Inc. and not to any of its subsidiaries.

We may issue warrants for the purchase of debt securities, preferred stock, common stock, other securities of the Company or any combination of the foregoing. Warrants may be issued alone or together with securities offered by any prospectus supplement and may be attached to, or separate from, those securities. We will issue each series of warrants under a separate warrant agreement and/or warrant certificate. The following outlines some of the general terms and provisions of the warrants. Further terms of the warrants and the applicable warrant agreement and/or warrant certificate will be stated in the applicable prospectus supplement. The following description and any description of the warrants in a prospectus supplement are not complete and are subject to and qualified in their entirety by reference to the terms and provisions of the warrant agreement and/or warrant certificate, which we will file with the SEC in connection with an issuance of any warrants.

The applicable prospectus supplement will include some or all of the following information:

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Before exercising their warrants, holders of warrants will not have any of the rights of holders of the securities purchasable upon such exercise, including the right to receive dividends, if any, or payments upon our liquidation, dissolution or winding-up, or to exercise voting rights, if any.

Description of Depositary Shares

In this section “Description of Depositary Shares,” the “Company,” “we,” “our,” or “us” refer only to Old Second Bancorp, Inc. and not to any of its subsidiaries.

The following is a summary of the general terms of the deposit agreement to govern any depositary shares we may offer representing fractional interests in shares of our preferred stock, the depositary shares themselves and the related depositary receipts. A copy of the deposit agreement and form of depositary receipt relating to any depositary shares we issue will be filed with the SEC as an exhibit to the registration statement of which this prospectus is a part or as an exhibit to a filing incorporated by reference in the registration statement. The specific terms of any depositary shares we may offer will be described in the applicable prospectus supplement. If so described in the applicable prospectus supplement, the terms of that series of depositary shares may differ from the general description of terms presented below. The following description and any description in a prospectus supplement is a summary only and is subject to, and qualified in its entirety by reference to the terms and provisions of the deposit agreement(s), and form of depositary receipt, which we will file with the SEC in connection with an issuance of depositary shares.

General

We may offer fractional interests in shares of our preferred stock, rather than full shares of preferred stock, most likely in the event that the number our then authorized but yet undesignated shares of preferred stock is not sufficient to offer full shares of preferred stock. If we do, we will provide for the issuance by a depositary to the public of receipts for depositary shares, each of which will represent a fractional interest in a share of a particular series of preferred stock.

The shares of any series of preferred stock underlying the depositary shares will be deposited under a separate deposit agreement between us and a bank or trust company having its principal office in the U.S. and having a combined capital and surplus of such amount as may be set forth in the applicable prospectus supplement, which we refer to in this section as the depositary. We will name the depositary in the applicable prospectus supplement. Subject to the terms of the deposit agreement, each owner of a depositary share will have a fractional interest in all the rights and preferences of the preferred stock underlying the depositary share. Those rights include any dividend, voting, redemption, conversion and liquidation rights.

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The depositary shares will be evidenced by depositary receipts issued under the deposit agreement. If you purchase fractional interests in shares of the related series of preferred stock, you will receive depositary receipts as described in the applicable prospectus supplement.

Unless we specify otherwise in the applicable prospectus supplement, you will not be entitled to receive the whole shares of preferred stock underlying the depositary shares.

Dividend Rights

The depositary will distribute all cash dividends or other cash distributions in respect of the preferred stock underlying the depositary shares to each record holder of depositary shares based on the number of depositary shares owned by that holder on the relevant record date. The depositary will distribute only that amount which can be distributed without attributing to any holder of depositary shares a fraction of one cent, and any balance not so distributed will be added to and treated as part of the next sum received by the depositary for distribution to record holders of depositary shares.

If there is a distribution other than in cash, the depositary will distribute property to the entitled record holders of depositary shares, unless the depositary determines that it is not feasible to make that distribution. In that case the depositary may, with our approval, adopt the method it deems equitable and practicable for making that distribution, including any sale of property and distribution of the net proceeds from this sale to the applicable holders.

The deposit agreement will also contain provisions relating to how any subscription or similar rights offered by us to holders of the preferred stock will be made available to the holders of depositary shares.

Voting Rights

When the depositary receives notice of any meeting at which the holders of the preferred stock may vote, the depositary will mail information about the meeting contained in the notice, and any accompanying proxy materials, to the record holders of the depositary shares relating to the preferred stock. Each record holder of such depositary shares on the record date, which will be the same date as the record date for the preferred stock, will be entitled to instruct the depositary as to how the preferred stock underlying the holder’s depositary shares should be voted.

Conversion or Exchange Rights

If any series of preferred stock underlying the depositary shares is subject to conversion or exchange, the applicable prospectus supplement will describe the rights or obligations of each record holder of depositary receipts to convert or exchange the depositary shares.

Redemption

If the series of the preferred stock underlying the depositary shares is subject to redemption, all or a part of the depositary shares will be redeemed from the redemption proceeds of that series of the preferred stock held by the depositary. The redemption price per depositary share will bear the same relationship to the redemption price per share of preferred stock that the depositary share bears to the underlying preferred stock. Whenever we redeem preferred stock held by the depositary, the depositary will redeem, as of the same redemption date, the number of depositary shares representing the preferred stock redeemed. If less than all the depositary shares are to be redeemed, the depositary shares to be redeemed will be selected by lot or pro rata as determined by the depositary.

After the date fixed for redemption, the depositary shares called for redemption will no longer be outstanding. When the depositary shares are no longer outstanding, all rights of the holders will cease, except the right to receive money or other property that the holders of the depositary shares were entitled to receive upon the redemption. Payments will be made when holders surrender their depositary receipts to the depositary.

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Taxation

Owners of depositary shares will be treated for U.S. federal income tax purposes as if they were owners of the preferred stock represented by the depositary shares. If necessary, the applicable prospectus supplement will provide a description of U.S. federal income tax consequences relating to the purchase and ownership of the depositary shares and the preferred stock represented by the depositary shares.

Amendment and Termination of the Deposit Agreement

The form of depositary receipt evidencing the depositary shares and any provision of the deposit agreement may be amended by agreement between us and the depositary at any time. However, certain amendments as specified in the applicable prospectus supplement will not be effective unless approved by the record holders of at least a majority of the depositary shares then-outstanding. A deposit agreement may be terminated by us or the depositary only if:

Charges of Depositary

We will pay all transfer and other taxes and governmental charges arising solely from the existence of the depositary arrangements. We will pay associated charges of the depositary for the initial deposit of the preferred stock and any redemption of the preferred stock. Holders of depositary shares will pay transfer and other taxes and governmental charges and any other charges that are stated to be their responsibility in the deposit agreement.

Resignation and Removal of Depositary

The depositary may resign at any time by delivering notice to us. We may also remove the depositary at any time. Resignations or removals will take effect when a successor depositary is appointed, and it accepts the appointment.

Description of Subscription Rights

In this section “Description of Subscription Rights,” the “Company,” “we,” “our,” or “us” refer only to Old Second Bancorp, Inc. and not to any of its subsidiaries.

The following is a summary of the general terms of the subscription rights to purchase common stock or other securities that we may offer to stockholders using this prospectus.

Subscription rights may be issued independently or together with any other security and may or may not be transferable. As part of any subscription rights offering, we may enter into a standby underwriting or other arrangement under which the underwriters or any other person would purchase any securities that are not purchased in such subscription rights offering. If we issue subscription rights, they will be governed by a separate subscription agent agreement that we will sign with a bank or trust company, as rights agent, that will be named in the applicable prospectus supplement. The rights agent will act solely as our agent and will not assume any obligation to any holders of subscription rights certificates or beneficial owners of subscription rights.

The prospectus supplement relating to any subscription rights we offer will describe the specific terms of the offering and the subscription rights, including the record date for stockholders entitled to the subscription rights distribution, the number of subscription rights issued and the number of shares of common stock or other securities that may be purchased upon exercise of the subscription rights, the exercise price of the subscription rights, the date on which the subscription rights will become effective and the date on which the subscription rights will expire, and

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any material U.S. federal income tax considerations. The following description and any description in a prospectus supplement is a summary only and is subject to, and qualified in its entirety by reference to the terms and provisions of the subscription rights, which we will file with the SEC in connection with an issuance of subscription rights.

In general, a subscription right entitles the holder to purchase for cash a specific number of shares of common stock or other securities at a specified exercise price. The rights are normally issued to stockholders as of a specific record date, may be exercised only for a limited period of time and become void following the expiration of such period. If we determine to issue subscription rights, we will accompany this prospectus with a prospectus supplement that will describe, among other things:

If fewer than all of the subscription rights issued in any rights offering are exercised, we may offer any unsubscribed securities directly to persons other than stockholders, to or through agents, underwriters or dealers or through a combination of such methods, including pursuant to standby arrangements, as described in the applicable prospectus supplement. After the close of business on the expiration date of a subscription rights offering, all unexercised subscription rights will become void.

Description of Stock Purchase Contracts and Stock Purchase Units

In this section “Description of Stock Purchase Contracts and Stock Purchase Units,” the “Company,” “we,” “our,” or “us” refer only to Old Second Bancorp, Inc. and not to any of its subsidiaries.

We may issue stock purchase contracts, including contracts obligating holders to purchase from us, and us to sell to the holders, a fixed or varying number of shares of our common stock, preferred stock, depositary shares or debt securities at a future date or dates, which we refer to in this prospectus as “Stock Purchase Contracts.” The price per share, and number of shares, of our common stock, preferred stock, depositary shares or debt securities may be fixed at the time the Stock Purchase Contracts are issued or may be determined by reference to a specific formula set forth in the Stock Purchase Contracts. The Stock Purchase Contracts may be issued separately or as a part of units consisting of a Stock Purchase Contract and our debt securities or debt obligations of third parties, including Treasury securities, securing the holders’ obligations to purchase the shares of our common stock under the Stock Purchase Contracts, which we refer to in this prospectus as “Stock Purchase Units.” The Stock Purchase Contracts may require holders to secure their obligations thereunder in a specified manner. The Stock Purchase Contracts also may require us to make periodic payments to the holders of the Stock Purchase Units or vice-versa and such payments may be unsecured or prefunded on some basis.

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The applicable prospectus supplement will describe the terms of any Stock Purchase Contracts or Stock Purchase Units. Any description in a prospectus supplement is a summary only and is subject to, and qualified in its entirety by reference to the terms and provisions of the Stock Purchase Contracts, and, if applicable, collateral or depositary arrangements, relating to the Stock Purchase Contracts or Stock Purchase Units, which we will file with the SEC in connection with an issuance of the Stock Purchase Contracts or Stock Purchase Units. Material U.S. federal income tax considerations applicable to the Stock Purchase Units and the Stock Purchase Contracts will also be discussed in the applicable prospectus supplement.

Description of Units

In this section “Description of Units,” the “Company,” “we,” “our,” or “us” refer only to Old Second Bancorp, Inc. and not to any of its subsidiaries.

As specified in the applicable prospectus supplement, we may issue units consisting of one or more debt securities, shares of common stock, shares of preferred stock, depositary shares, warrants or other securities, or any combination of such securities, including guarantees of any securities.

A prospectus supplement and any other offering materials relating to any units issued under the registration statement containing this prospectus will specify the terms of the units, including:

Plan of Distribution

We may sell the securities covered by this prospectus from time to time at market prices prevailing at the time of sale, at prices related to such prevailing market prices at the time of sale, at negotiated prices or at fixed prices, which may change from time to time. We may sell the securities directly to one or more purchasers, through agents, to dealers, through underwriters, brokers or dealers, or through a combination of any of these sales methods or through any other method permitted by law (including in “at the market” equity offerings as defined in Rule 415 of the Securities Act of 1933, as amended, or the “Securities Act”). We reserve the right to accept or reject, in whole or in part, any proposed purchase of securities, whether the purchase is to be made directly or through agents.

Each time that we use this prospectus to sell our securities, we will also provide a prospectus supplement, if required, that contains the specific terms of the offering, including:

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Only underwriters that we have named in a prospectus supplement will be underwriters of the securities offered by that prospectus supplement.

If underwriters are used in the sale, they will acquire the securities for their own account and may resell the securities from time to time in one or more transactions at a fixed public offering price or at varying prices determined at the time of sale. The obligations of the underwriters to purchase the securities will be subject to the conditions set forth in the applicable underwriting agreement. We may offer the securities to the public through underwriting syndicates represented by managing underwriters or by underwriters without a syndicate. Subject to certain conditions, the underwriters will be obligated to purchase all of the securities offered by the prospectus supplement, other than securities covered by any over-allotment option. Any public offering price and any discounts or concessions allowed or reallowed or paid to dealers may change from time to time. We may use underwriters with whom we have a material relationship. We will describe in the prospectus supplement, naming the underwriter, the nature of any such relationship.

We may sell securities directly or through agents we designate from time to time. We will name any agent involved in the offering and sale of securities and we will describe any commissions we will pay the agent in the prospectus supplement. Unless the prospectus supplement states otherwise, our agent will act on a best-efforts basis for the period of its appointment.

We may authorize agents or underwriters to solicit offers by certain types of institutional investors to purchase securities from us at the public offering price set forth in the prospectus supplement pursuant to delayed delivery contracts providing for payment and delivery on a specified date in the future. We will describe the conditions to these contracts and the commissions we must pay for solicitation of these contracts in the applicable prospectus supplement.

We may provide agents and underwriters with indemnification against civil liabilities, including liabilities under the Securities Act, or contribution with respect to payments that the agents or underwriters may make with respect to these liabilities. Agents and underwriters may engage in transactions with, or perform services for, us in the ordinary course of business.

All securities we may offer, other than common stock or other outstanding securities, will be new issues of securities with no established trading market. Any underwriters may make a market in these securities but will not be obligated to do so and may discontinue any market making at any time without notice. We cannot guarantee the liquidity of the trading markets for any securities.

Any underwriter may engage in over-allotment, stabilizing transactions, short-covering transactions and penalty bids in accordance with Regulation M under the Exchange Act. Over-allotment involves sales in excess of the offering size, which create a short position. Stabilizing transactions permit bids to purchase the underlying security so long as the stabilizing bids do not exceed a specified maximum price. Syndicate-covering or other short-covering transactions involve purchases of the securities, either through exercise of the over-allotment option or in the open market after the distribution is completed, to cover short positions. Penalty bids permit the underwriters to reclaim a selling concession from a dealer when the securities originally sold by the dealer are purchased in a stabilizing or covering transaction to cover short positions. Those activities may cause the price of the securities to be higher than it would otherwise be. If commenced, the underwriters may discontinue any of the activities at any time.

Under the securities laws of some states, to the extent applicable, the securities may be sold in such states only through registered or licensed brokers or dealers. In addition, if our common stock is no longer listed on the NASDAQ Global Select Market or another national securities exchange, in some states the securities may not be sold unless such securities have been registered or qualified for sale in such state or an exemption from registration or qualification is available and is complied with.

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Legal Matters

Certain legal matters in connection with any offering of securities made by this prospectus will be passed upon for us by our counsel Nelson Mullins Riley & Scarborough LLP of Atlanta, Georgia. If the securities are being distributed in an underwritten offering, certain legal matters will be passed upon for the underwriters by counsel identified in the related prospectus supplement.

Experts

The financial statements incorporated in this prospectus by reference to the Annual Report on Form 10-K for the year ended December 31, 2022, and the effectiveness of our internal controls over financial reporting as of December 31, 2022, have been so incorporated in reliance on the report of Plante & Moran PLLC, independent registered public accounting firm, given on their authority as experts in auditing and accounting.

Where You Can Find More Information

We are subject to the informational requirements of the Exchange Act and file with the SEC proxy statements, Annual Reports on Form 10-K, Quarterly Reports on Form 10-Q and Current Reports on Form 8-K. Our SEC filings are also available to the public from the SEC’s web site at www.sec.gov or on our website at www.oldsecond.com. However, other than our available SEC filings, the information on, or that can be accessible through, our website does not constitute a part of, and is not incorporated by reference in, this prospectus.

This prospectus is part of a registration statement on Form S-3 filed by us with the SEC under the Securities Act. As permitted by the SEC, this prospectus does not contain all the information in the registration statement filed with the SEC. For a more complete understanding of this offering, you should refer to the complete registration statement, including exhibits, on Form S-3 that may be obtained as described above. Statements contained in this prospectus about the contents of any contract or other document are not necessarily complete. If we have filed any contract or other document as an exhibit to the registration statement or any other document incorporated by reference in the registration statement, you should read the exhibit for a more complete understanding of the contract or other document or matter involved. Each statement regarding a contract or other document is qualified in its entirety by reference to the actual contract or other document.

Incorporation of Certain Information By Reference

The SEC allows us to “incorporate by reference” into this prospectus the information that we file with it, which means that we can disclose important information to you by referring you to other documents. The information incorporated by reference is an important part of this prospectus. Information incorporated by reference is considered to be part of this prospectus, except for any information that is superseded by information included directly in this prospectus. Any statement contained in this prospectus or a document incorporated by reference in this prospectus will be deemed to be modified or superseded for purposes of this prospectus to the extent that a statement contained in this prospectus or in any other subsequently filed document that is incorporated by reference in this prospectus modifies or supersedes the statement. Any statement so modified or superseded will not be deemed, except as so modified or superseded, to constitute a part of this prospectus.

We incorporate by reference into this prospectus the documents listed below and any future filings we make with the SEC under Sections 13(a), 13(c), 14 or 15(d) of the Exchange Act after the date of this prospectus and prior to the termination of this offering as well as any filings after the date of the initial registration statement of which this prospectus forms a part and prior to the effectiveness of such registration statement, but excluding, in each case, information “furnished” rather than “filed” in accordance with SEC rules (unless otherwise indicated therein):

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We will provide to each person, including any beneficial owner, to whom a prospectus is delivered, without charge, upon written or oral request, a copy of any or all of the documents that are incorporated by reference into this prospectus and a copy of any or all other contracts or documents which are referred to in this prospectus. Requests should be directed to:

Old Second Bancorp, Inc.

Attention: Corporate Secretary

37 South River Street

Aurora, Illinois 60507

Telephone number: (630) 892-0202

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PART II
INFORMATION NOT REQUIRED IN PROSPECTUS

Item 14. Other Expenses of Issuance and Distribution

The following table sets forth our estimated expenses to be incurred in connection with the issuance and distribution of the securities being registered, other than underwriting discounts and commissions. We will pay all of these expenses.

Item 15. Indemnification of Directors and Officers

Delaware Law. Section 145 of the Delaware General Corporation Law permits a corporation to indemnify any person who was or is a party or is threatened to be made a party to any threatened, pending or completed action, suit or proceeding, whether civil, criminal, administrative or investigative, by reason of the fact that the person is or was a director, officer, employee or agent of the corporation or another enterprise if serving at the request of the corporation. Depending on the character of the proceeding, a corporation may indemnify against expenses (including attorneys’ fees), judgments, fines and amounts paid in settlement actually and reasonably incurred in connection with such action, suit or proceeding if the person indemnified acted in good faith and in a manner reasonably believed to be in or not opposed to the best interests of the corporation and, with respect to any criminal action or proceeding, had no reasonable cause to believe the person’s conduct was unlawful. In the case of an action by or in the right of the corporation, no indemnification may be made with respect to any claim, issue or matter as to which such person shall have been adjudged to be liable to the corporation unless and only to the extent that the Delaware Court of Chancery or the court in which such action or suit was brought shall determine that, despite the adjudication of liability, such person is fairly and reasonably entitled to indemnity for such expenses which the court shall deem proper. Section 145 further provides that to the extent a director or officer of a corporation has been successful on the merits or in the defense of any action, suit or proceeding referred to above, or in the defense of any claim, issue or matter therein, he or she shall be indemnified against expenses (including attorneys’ fees) actually and reasonably incurred by him or her in connection therewith.

Certificate of Incorporation and Bylaws. Article VIII of our Certificate of Incorporation provides that we shall, to the full extent permitted by Section 145 of the Delaware General Corporation Law, indemnify all persons whom we may indemnify pursuant thereto. Section 8.8 of our Bylaws contains indemnification provisions substantially similar to Section 145 of the Delaware General Corporation Law.

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Liability Insurance. We have obtained directors’ and officers’ liability insurance. The policy provides for coverage including prior acts and liabilities under the Securities Act, within the limits and subject to the limitations of such insurance.

Item 16. Exhibits

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The undersigned registrant hereby undertakes:

provided, however, that paragraphs (1)(i), (1)(ii) and (1)(iii) of the above do not apply if the information required to be included in a post-effective amendment by those paragraphs is contained in reports filed with or furnished to the SEC by the registrant pursuant to Section 13 or

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Section 15(d) of the Securities Exchange Act of 1934, as amended, that are incorporated by reference in this registration statement, or is contained in a form of prospectus filed pursuant to Rule 424(b) that is part of the registration statement.

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Insofar as indemnification for liabilities arising under the Securities Act of 1933, as amended, may be permitted to directors, officers and controlling persons of the registrant pursuant to the indemnification provisions described in Item 15 above, or otherwise, the registrant has been advised that in the opinion of the SEC such indemnification is against public policy as expressed in the Securities Act of 1933, as amended, and is, therefore, unenforceable. In the event that a claim for indemnification against such liabilities (other than the payment by the registrant of expenses incurred or paid by a director, officer or controlling person of the registrant in the successful defense of any action, suit or proceeding) is asserted by such director, officer or controlling person in connection with the securities being registered, the registrant will, unless in the opinion of its counsel the matter has been settled by controlling precedent, submit to a court of appropriate jurisdiction the question whether such indemnification by it is against public policy as expressed in the Securities Act of 1933, as amended, and will be governed by the final adjudication of such issue .

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SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, the registrant certifies that it has reasonable grounds to believe that it meets all of the requirements for filing on Form S-3 and has duly caused this registration statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the city of Aurora, state of Illinois, on the 24th day of August, 2023.

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POWER OF ATTORNEY

Pursuant to the requirements of the Securities Act of 1933, this registration statement has been signed below by the following persons in the capacities indicated on August 24, 2023.

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* By: /s/ Bradley S. Adams

Bradley S. Adams

Attorney-in-Fact

August 24, 2023