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Loans - Inclusion (Details) (Minimum, USD $)	3 Months Ended
	Mar. 31, 2015
Minimum
Loans by risk rating
Loan commitment for inclusion in credit quality analysis	$ 50,000osbc_OutstandingLoanCommitmentsThresholdForCreditQualityCategorizing / us-gaap_RangeAxis = us-gaap_MinimumMember

X

- Definition

The outstanding loan commitment to be included in credit quality indicator analysis.

+ References

No definition available.

+ Details

Name:	osbc_OutstandingLoanCommitmentsThresholdForCreditQualityCategorizing
Namespace Prefix:	osbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration