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Loans - Inclusion (Details)	12 Months Ended
	Dec. 31, 2017 USD ($)
Minimum
Loans by risk rating
Loan commitment for inclusion in credit quality analysis	$ 50,000

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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

X

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: