Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2023	Aug. 04, 2023
Document and Entity Information
Document Type	10-Q
Document Quarterly Report	true
Document Transition Report	false
Document Period End Date	Jun. 30, 2023
Entity File Number	000-10537
Entity Registrant Name	OLD SECOND BANCORP INC
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	36-3143493
Entity Address, Address Line One	37 South River Street
Entity Address, City or Town	Aurora
Entity Address, State or Province	IL
Entity Address, Postal Zip Code	60507
City Area Code	630
Local Phone Number	892-0202
Title of 12(b) Security	Common Stock
Trading Symbol	OSBC
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		44,675,057
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2023
Document Fiscal Period Focus	Q2
Entity Central Index Key	0000357173
Amendment Flag	false