OLD SECOND BANCORP INC

We will bear the cost of this proxy solicitation. Solicitation will be made primarily through the use of the mail, but our officers, directors or employees may solicit proxies personally, by telephone or through any other mode of communication, without additional remuneration to our officers, directors or employees for such activity. In addition, we will reimburse brokerage houses and other custodians, nominees or fiduciaries for their reasonable expenses in forwarding proxy materials to the beneficial owner of such shares.

As of the date of this proxy statement, we do not know of any other matters to be brought before the annual meeting. However, if any other matters should properly come before the meeting, it is the intention of the persons named in the enclosed proxy to vote thereon in accordance with their best judgment.

SCAN TO VIEW MATERIALS & VOTE OLD SECOND BANCORP, INC. ATTN: SHIRLEY CANTRELL 37 SOUTH RIVER STREET AURORA, IL 60506-4172 VOTE BY INTERNET Before The Meeting - www.proxyvote.com or scan the QR Barcode above Use the Internet to transmit your voting instructions and for electronic delivery of information. Vote by 11:59 P.M. ET on May 20, 2024. Have your proxy card in hand when you access the web site and follow the instructions to obtain your records and to create an electronic voting instruction form. During The Meeting - Go to www.virtualshareholdermeeting.com/OSBC2024 You may attend the meeting via the Internet and vote during the meeting. Have the information that is printed in the box marked by the arrow available and follow the instructions. VOTE BY PHONE - 1-800-690-6903 Use any touch-tone telephone to transmit your voting instructions. Vote by 11:59 P.M. ET on May 20, 2024. Have your proxy card in hand when you call and then follow the instructions. VOTE BY MAIL Mark, sign and date your proxy card and return it in the postage-paid envelope we have provided or return it to Vote Processing, c/o Broadridge, 51 Mercedes Way, Edgewood, NY 11717. TO VOTE, MARK BLOCKS BELOW IN BLUE OR BLACK INK AS FOLLOWS: V34025-P06750 KEEP THIS PORTION FOR YOUR RECORDS THIS PROXY CARD IS VALID ONLY WHEN SIGNED AND DATED. DETACH AND RETURN THIS PORTION ONLY OLD SECOND BANCORP, INC. The Board of Directors recommends you vote FOR each director nominee in Proposal 1: 1. Proposal to elect four Class II director nominees to serve for a term expiring in 2027: Nominees: For Against Abstain 1a. James Eccher 1b. Barry Finn 1c. Dennis Klaeser 1d. Hugh McLean ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ The Board of Directors recommends you vote FOR Proposals 2 and 3. For Against Abstain 2. Proposal to approve on a non-binding, advisory basis, the compensation of the Company's named executive officers, as described in the Company's ​ ​ ​ Proxy Statement. 3. Proposal to ratify the appointment of Plante & Moran, PLLC as the Company's independent registered public accountants for the fiscal year ending ​ ​ ​ December 31, 2024. NOTE: Such other business as may properly come before the meeting or any adjournment thereof. PLEASE SIGN AND DATE YOUR PROXY CARD. PLEASE SIGN IT EXACTLY AS YOUR NAME OR NAMES APPEAR(S) ABOVE. ALL JOINT OWNERS OF SHARES SHOULD SIGN. STATE THE FULL TITLE WHEN SIGNING AS EXECUTOR, ADMINISTRATOR, TRUSTEE, GUARDIAN, ETC. PLEASE RETURN YOUR SIGNED PROXY CARD IN THE ENCLOSED ENVELOPE. Signature [PLEASE SIGN WITHIN BOX] Date Signature (Joint Owners) Date

Important Notice Regarding the Availability of Proxy Materials for the Annual Meeting: The Proxy Statement and Annual Report are available at www.proxyvote.com. V34026-P06750 ANNUAL MEETING OF STOCKHOLDERS OF OLD SECOND BANCORP, INC. TO BE HELD ON MAY 21, 2024 This proxy is solicited on behalf of the Board of Directors The undersigned hereby appoints Gary Collins and Bradley Adams, or any of them, with full power to act alone, the true and lawful proxies of the undersigned, each with full power of substitution and revocation, and hereby authorizes them to represent and to vote all of the shares of common stock of Old Second Bancorp, Inc. (the "Company"), that the undersigned is entitled to vote, as fully as the undersigned could do if personally present, at the Annual Meeting of Stockholders of the Company to be held virtually via live audio webcast at www.virtualshareholdermeeting.com/OSBC2024 on Tuesday, May 21, 2024 at 9:00 a.m. Central Time, and at any adjournment or postponement thereof. This proxy, when properly executed, will be voted in the manner directed herein by the undersigned stockholder. If no direction is made and the proxy is signed and returned, this proxy will be voted "FOR" each director nominee listed in Proposal 1 and "FOR" Proposals 2 and 3, and the named proxies will vote on any other business that properly comes before the Annual Meeting or any postponement or adjournment of it in accordance with their discretion. The undersigned acknowledges receipt of the Notice and Proxy Statement relating to the Annual Meeting of Stockholders. Continued and to be signed on reverse side