Loans - TDR (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015 contract	Mar. 31, 2014 contract
Troubled debt restructurings - modified during the period
# of contracts	2	4
Pre-modification recorded investment	$ 447	$ 1,659
Post-modification recorded investment	338	1,394
TDR's defaulted
# of contracts	0
Real estate - commercial \| Other
Troubled debt restructurings - modified during the period
# of contracts		2
Pre-modification recorded investment		1,320
Post-modification recorded investment		1,191
Real estate - commercial \| Bifurcate
Troubled debt restructurings - modified during the period
# of contracts	1
Pre-modification recorded investment	300
Post-modification recorded investment	182
Real estate - residential \| Owner occupied \| Other
Troubled debt restructurings - modified during the period
# of contracts	1
Pre-modification recorded investment	147
Post-modification recorded investment	156
Real estate - residential \| Owner occupied \| HAMP
Troubled debt restructurings - modified during the period
# of contracts		1
Pre-modification recorded investment		102
Post-modification recorded investment		76
Real estate - residential \| Owner occupied \| Deferral
Troubled debt restructurings - modified during the period
# of contracts		1
Pre-modification recorded investment		237
Post-modification recorded investment		$ 127

Name:	osbc_FinancingReceivableModificationsSubsequentPaymentDefaultAbstract
Namespace Prefix:	osbc_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

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

- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 33

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Post-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Pre-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Number of financing receivables that have been modified by troubled debt restructurings within the previous 12 months and for which there was a payment default.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 34

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953682-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Details

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

- Details

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