Sec Form 4 Filing - Campbell Darin Patrick @ OLD SECOND BANCORP INC - 2025-07-18

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

OMB APPROVAL OMB Number: 3235-0287 Expires: November 30, 2011 Estimated average burden hours per response... 0.5

1. Name and Address of Reporting Person Campbell Darin Patrick 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 X __ Officer (give title below) _____ Other (specify below) President, Powersports Lending (Last) (First) (Middle) 37 S. RIVER ST. 3. Date of Earliest Transaction (MM/DD/YY) 07/18/2025 (Street) AURORA, IL60506 4. If Amendment, Date Original Filed (MM/DD/YY) 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 (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 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 07/18/2025 A 57,565 ( 1 ) A $ 0 57,565 I IRA Old Second Bancorp, Inc. Common Stock 309,680 ( 2 ) D

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 (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 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 (MM/DD/YY) 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 Campbell Darin Patrick 37 S. RIVER ST. AURORA, IL60506 X President, Powersports Lending Signatures /s/ Shirley Cantrell, Attorney-in-Fact 07/21/2025 Signature of Reporting Person Date

Explanation of Responses:

( 1 )These shares were issuable pursuant to the merger of Bancorp Financial, Inc. with and into Old Second Bancorp, Inc., which became effective on July 1, 2025. The reporting person held 22,300 shares of Bancorp Financial, Inc., which pursuant to the merger agreement was converted into 57,565 shares of Old Second Bancorp, Inc. common stock. These shares are held in a custodial account in an IRA on behalf of the reporting person. The exchange of shares occurred on July 19, 2025. ( 2 )These securities are issuable pursuant to the merger of Bancorp Financial, Inc. with and into Old Second Bancorp, Inc., which became effective on July 1, 2025. The reporting person held 119,966 shares of Bancorp Financial, Inc., which pursuant to the merger agreement, will be converted into 309,680 shares of Old Second Bancorp, Inc. common stock. The exchange of shares has not yet occurred, but the right to receive the shares is fixed and irrevocable under the merger agreement. The exchange of shares occurred on July 18, 2025.

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

* 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).

Note: File three copies of this Form, one of which must be manually signed. If space is insufficient, see Instruction 6 for procedure.
Persons who respond to the collection of information contained in this form are not required to respond unless the form displays a currently valid OMB Number.