Document and Entity Information - shares	3 Months Ended	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2020	Nov. 04, 2020
Document and Entity Information
Document Type		10-Q
Document Quarterly Report		true
Document Transition Report		false
Document Period End Date		Sep. 30, 2020
Entity File Number		0-10537
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			29,355,022
Current Fiscal Year End Date		--12-31
Document Fiscal Year Focus		2020
Document Fiscal Period Focus		Q3
Entity Central Index Key	0000357173
Amendment Flag		false