Summary of Significant Accounting Policies - Dividend (Details)	12 Months Ended
Dec. 31, 2016
90-Days or Greater Past Due Loans
Period from due date after which accrual of interest income is discontinued	90 days

Represents the period from due date after which accrual of interest income is discontinued when full loan repayment is in doubt or when a loan becomes contractually past due.

No definition available.

Name:	osbc_PeriodFromDueDateAfterWhichAccrualOfInterestIncomeIsDiscontinued
Namespace Prefix:	osbc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration