As filed with the Securities and Exchange Commission on April 1, 2014

Registration No. 333-193424

UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

Amendment No. 3
to

FORM S-1
REGISTRATION STATEMENT
UNDER
THE SECURITIES ACT OF 1933

OLD SECOND BANCORP, INC.
(Exact name of registrant as specified in its charter)

37 South River Street
Aurora, Illinois 60506
(630) 892-0202
(Address, including zip code and telephone number, including area code, of registrant's principal executive offices)

William B. Skoglund
Chairman and Chief Executive Officer
Old Second Bancorp, Inc.
37 South River Street
Aurora, Illinois 60506
(630) 892-0202
(Name, address, including zip code and telephone number, including area code, of agent for service)

Approximate date of commencement of proposed sale to the public: As soon as practicable after this registration statement becomes effective.

           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, check the following box.    o

           If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) 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 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 post-effective amendment filed pursuant to Rule 462(d) 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

           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" and "smaller reporting company" in Rule 12b-2 of the Securities Exchange Act of 1934, as amended (the "Exchange Act"). (Check one):

CALCULATION OF REGISTRATION FEE

(1)

Estimated solely for the purpose of calculating the registration fee pursuant to Rule 457(o) under the Securities Act.

(2)

Includes offering price of shares that the underwriters have the option to purchase to cover over-allotments, if any.

(3)

Each share of Old Second Bancorp, Inc., common stock has attached thereto the right to purchase one one-thousandth (subject to adjustment) of a share of Series A Junior Participating Preferred Stock, $1.00 par value per share.

(4)

The Registrant previously paid a registration fee of $10,047 for the registration of $78,000,000 worth of Common Stock.

           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 or until the Registration Statement shall become effective on such date as the Securities and Exchange Commission, acting pursuant to said Section 8(a), may determine.

Explanatory Note

        This Amendment No. 3 is being filed solely for the purpose of amending Exhibit 23.1, which was previously filed with Amendment No. 2 to Form S-1/A on March 26, 2014. No changes are being made to the prospectus constituting Part I of the registration statement or Items 13, 14, 15 and 17 of Part II of the registration statement.

PART II

INFORMATION NOT REQUIRED IN PROSPECTUS

Item 13.    Other Expenses of Issuance and Distribution.

        The following table sets forth the costs and expenses, other than underwriting commissions, to be paid in connection with the sale of shares of our common stock being registered, all of which will be paid by us. We will pay all of these expenses. All expenses other than the SEC registration fee and FINRA filing fee are estimates, pursuant to the instruction to Item 511 of Regulation S-K, subject to future contingencies, of the expenses to be incurred by us in connection with the issuance and distribution of the securities being registered.

Item 14.    Indemnification of Directors and Officers.

        Section 145 of the DGCL authorizes a corporation's board of directors to grant indemnification to directors and officers in terms sufficiently broad to permit such indemnification under certain circumstances for liabilities, including reimbursement for expenses incurred, arising under the Securities Act.

        Our certificate of incorporation provides that we must indemnify, to the fullest extent permitted by Section 145 of the DGCL, our directors and officers. Under Delaware law, a Delaware corporation may 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 (other than an action by or in the right of the corporation) by reason of the fact that he or she is or was a director, officer, employee or agent of the corporation, or is or was serving at the request of the corporation as a director, officer, employee or agent of another corporation, partnership, joint venture, trust, employee benefit plan or other enterprise, against expenses (including attorneys' fees), judgments, fines and amounts paid in settlement actually and reasonably incurred by him or her in connection with such action, suit or proceeding if he or she acted in good faith and in a manner he or she 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 his or her conduct was unlawful.

        Our certificate of incorporation also includes a provision that eliminates the personal liability of our directors for monetary damages for breach of their fiduciary duty as a director to the fullest extent permitted by the DGCL, except for liability for any breach of the director's duty of loyalty to us or our shareholders; for acts or omissions not in good faith or that involve intentional misconduct or a knowing violation of law; under Section 174 of the DGCL, which relates to unlawful payment of dividends or unlawful stock purchase or redemption and expressly sets forth a negligence standard with respect to such liability; and for any transaction from which the director derived an improper personal benefit.

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Item 16.    Exhibits.

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Item 17.    Undertakings.

        Insofar as indemnification for liabilities arising under the Securities Act of 1933 (the "Securities Act") may be permitted to directors, officers and controlling persons of the registrant pursuant to the indemnification provisions described herein, or otherwise, the registrant has been advised that in the opinion of the Securities and Exchange Commission such indemnification is against public policy as expressed in the Securities Act 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 and will be governed by the final adjudication of such issue.

        The undersigned registrant hereby undertakes to deliver or cause to be delivered with the prospectus, to each person to whom the prospectus is sent or given, the latest annual report, to security holders that is incorporated by reference in the prospectus and furnished pursuant to and meeting the requirements of Rule 14a-3 or Rule 14c-3 under the Securities Exchange Act of 1934; and, where interim financial information required to be presented by Article 3 of Regulation S-X is not set forth in the prospectus, to deliver, or cause to be delivered to each person to whom the prospectus is sent or given, the latest quarterly report that is specifically incorporated by reference in the prospectus to provide such interim financial information.

        The undersigned registrant hereby undertakes that:

        (1)   For purposes of determining any liability under the Securities Act of 1933, the information omitted from the form of prospectus filed as part of this registration statement in reliance upon Rule 430A and contained in a form of prospectus filed by the registrant pursuant to Rule 424(b)(1) or (4) or 497(h) under the Securities Act shall be deemed to be part of this registration statement as of the time it was declared effective.

        (2)   For the purpose of determining any liability under the Securities Act of 1933, each post-effective amendment that contains a form of prospectus shall be deemed to be a new registration statement relating to the securities offered therein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof.

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SIGNATURES

        Pursuant to the requirements of the Securities Act of 1933, the registrant has duly caused this Amendment No. 3 to this registration statement to be signed on its behalf by the undersigned thereunto duly authorized, in the City of Aurora, State of Illinois, on April 1, 2014.

POWER OF ATTORNEY

        Pursuant to the Securities Act, this Amendment No. 3 to this registration statement has been signed below by the following persons in the capacities and on the dates indicated.

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EXHIBIT INDEX

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Item 13. Other Expenses of Issuance and Distribution.
Item 14. Indemnification of Directors and Officers.
Item 16. Exhibits.
Item 17. Undertakings.