Statement of Changes in Beneficial Ownership (Form 4)

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b). Check this box to indicate that a transaction was made pursuant to a contract, instruction or written plan that is intended to satisfy the affirmative defense conditions of Rule 10b5-1(c). See Instruction 10.

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP OF SECURITIES Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934 or Section 30(h) of the Investment Company Act of 1940

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(Print or Type Responses)

1. Name and Address of Reporting Person * YORK JILL E 2. Issuer Name and Ticker or Trading Symbol OLD SECOND BANCORP INC [OSBC] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) __X__ Director _____ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) 37 S. RIVER ST. 3. Date of Earliest Transaction (Month/Day/Year) 05/11/2021 (Street) AURORA, IL 60506 4. If Amendment, Date Original Filed (Month/Day/Year) 6. Individual or Joint/Group Filing (Check Applicable Line) _X_ Form filed by One Reporting Person ___ Form filed by More than One Reporting Person (City) (State) (Zip) Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned

1.Title of Security (Instr. 3) 2. Transaction Date (Month/Day/Year) 2A. Deemed Execution Date, if any (Month/Day/Year) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Old Second Bancorp, Inc. Common Stock 05/11/2021 L 14.2083(1) A $14.08 4,319.6382(2) D Old Second Bancorp, Inc. Common Stock 08/11/2021 L 17.5446(1) A $11.44 4,319.6382(2) D Old Second Bancorp, Inc. Common Stock 11/09/2021 L 14.5974(1) A $13.81 4,319.6382(2) D Old Second Bancorp, Inc. Common Stock 02/08/2022 L 14.0252(1) A $14.43 4,319.6382(2) D Old Second Bancorp, Inc. Common Stock 05/10/2022 L 14.3579(1) A $14.14 4,319.6382(2) D Old Second Bancorp, Inc. Common Stock 08/09/2022 L 14.5777(1) A $13.98 4,319.6382(2) D Old Second Bancorp, Inc. Common Stock 11/08/2022 L 12.0515(1) A $16.97 4,319.6382(2) D Old Second Bancorp, Inc. Common Stock 02/07/2023 L 11.9166(1) A $17.21 4,319.6382(2) D Old Second Bancorp, Inc. Common Stock 05/09/2023 L 18.4285(1) A $11.16 4,319.6382(2) D Old Second Bancorp, Inc. Common Stock 08/08/2023 L 12.9172(1) A $15.99 4,319.6382(2) D Old Second Bancorp, Inc. Common Stock 11/07/2023 L 14.5225(1) A $14.27 4,319.6382(2) D Old Second Bancorp, Inc. Common Stock 02/06/2024 L 15.4522(1) A $13.46 4,319.6382(2) D Old Second Bancorp, Inc. Common Stock 05/06/2024 L 14.46(1) A $14.43 4,319.6382(2) D Old Second Bancorp, Inc. Common Stock 08/05/2024 L 14.0955(1) A $14.86 4,319.6382(2) D Old Second Bancorp, Inc. Common Stock 11/04/2024 L 15.6156(1) A $16.15 4,319.6382(2) D Old Second Bancorp, Inc. Common Stock 02/10/2025 L 13.3087(1) A $19.02 4,319.6382(2) D Old Second Bancorp, Inc. Common Stock 05/05/2025 L 15.5779(1) A $16.3 4,319.6382(2) D Old Second Bancorp, Inc. Common Stock 08/04/2025 L 15.3405(1) A $16.61 4,319.6382(2) D Old Second Bancorp, Inc. Common Stock 11/10/2025 L 16.2619(1) A $18.35 4,319.6382(2) D Old Second Bancorp, Inc. Common Stock 02/09/2026 L 14.1217(1) A $21.21 4,319.6382(2) D Old Second Bancorp, Inc. Common Stock 05/11/2026 L 14.3767(1) A $20.9 4,319.6382(2) D Old Second Bancorp, Inc. Common Stock 08/10/2026 L 11.8801(1) A $25.38 4,319.6382(2) D Old Second Bancorp, Inc. Common Stock 15,430(3) I Living Trust Restricted Stock Units 10,595 D

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Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned
(e.g., puts, calls, warrants, options, convertible securities)

1. Title of Derivative Security (Instr. 3)

2. Conversion or Exercise Price of Derivative Security

3. Transaction Date (Month/Day/Year)

3A. Deemed Execution Date, if any (Month/Day/Year)

4. Transaction Code (Instr. 8)

5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5)

6. Date Exercisable and Expiration Date (Month/Day/Year)

7. Title and Amount of Underlying Securities (Instr. 3 and 4)

8. Price of Derivative Security (Instr. 5)

9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4)

10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4)

11. Nature of Indirect Beneficial Ownership (Instr. 4)

Code

V

(A)

(D)

Date Exercisable

Expiration Date

Title

Amount or Number of Shares

Reporting Owners

Reporting Owner Name / Address	Relationships
	Director	10% Owner	Officer	Other
YORK JILL E 37 S. RIVER ST. AURORA, IL 60506	X

Signatures

/s/ Shirley Cantrell, Attorney-in-Fact		10/21/2026
**Signature of Reporting Person		Date

Explanation of Responses:

*	If the form is filed by more than one reporting person, see Instruction 4(b)(v).
**	Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).
(1)	Represents shares acquired through the automatic reinvestment of cash dividends paid on shares of the Issuer's common stock held in the Reporting Person's individual retirement account, under a dividend reinvestment feature offered by the account custodian. The custodian's reinvestment feature is not a plan sponsored by the Issuer. These acquisitions were not previously reported because of an administrative oversight, and are reported herein to correct the omission.
(2)	The amount of securities beneficially owned represents the Reporting Person's holdings in her individual retirement account as of the date hereof, after giving effect to all dividend reinvestment transactions reported herein. The transactions reported herein were not previously reported.
(3)	The Reporting Person additionally beneficially owns 15,430 shares through her living trust as of October 15, 2026. This amount reflects her current trust holdings and supersedes the trust balance previously reported.

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