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Loans - Major Classifications - Loan Concentrations (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Loans
Growth in loan balance	$ 279.4
Total real estate
Loans
Loans receivable as a percentage of total portfolio	77.90%	78.00%

X

- Definition

Represents the amount of increase in loan balance over previous year.

+ References

No definition available.

+ Details

Name:	osbc_IncreaseInLoans
Namespace Prefix:	osbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

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

+ References

No definition available.

+ Details

Name:	osbc_LoansAndLeasesReceivableAsPercentageOfAggregatePortfolio
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_FinancingReceivablePortfolioSegmentAxis=osbc_RealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: