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Loans - Major Classifications - Loan Concentrations (Details)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Total real estate
Loans
Loans receivable as a percentage of total portfolio	86.10%	87.60%

X

- Definition

Represents the percentage of total portfolio that is represented by a particular financial receivable category.

+ References

No definition available.

+ Details

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Namespace Prefix:	osbc_
Data Type:	num:percentItemType
Balance Type:	na
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_AccountsNotesAndLoansReceivableLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=osbc_RealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: