X0609 4/A 2026-09-03 2026-09-03 0000357173 OLD SECOND BANCORP INC OSBC 0001184764 MCLEAN HUGH H false 408 S. PROSPECT AVE ELMHURST IL 60126 true false false false 0 Old Second Bancorp, Inc. Common Stock 160351 D Restricted Stock Units 10595 D The Form 4 filed on September 3, 2026 reported a sale of 25,000 shares of the Issuer's common stock. On September 4, 2026, the reporting person's broker reversed the transaction through its broker error account after determining that it had not timely filed the required Form 144 in connection with the sale. The reporting person did not receive any proceeds from the transaction. This amended Form 4 deletes the previously reported sale and reflects the reporting person's corrected holdings. /s/ Shirley Cantrell, Attorney-in-Fact 2026-09-08