Loans - Impaired Loans - Recorded Investment, Unpaid Principal Balance and Related Allowance (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Recorded Investment
With no related allowance recorded	$ 25,016	$ 35,011
With an allowance recorded	6,910	875
Total impaired loans	31,926	35,886
Unpaid Principal Balance
With no related allowance recorded	30,106	41,003
With an allowance recorded	8,193	997
Total impaired loans	38,299	42,000
Related Allowance
With an allowance recorded	1,784	278
Commercial
Recorded Investment
With no related allowance recorded	1,897	1,500
Unpaid Principal Balance
With no related allowance recorded	2,571	2,114
Real estate - commercial \| Owner occupied general purpose
Recorded Investment
With no related allowance recorded	2,593	7,125
With an allowance recorded	4,011
Unpaid Principal Balance
With no related allowance recorded	3,279	7,870
With an allowance recorded	4,657
Related Allowance
With an allowance recorded	1,304
Real estate - commercial \| Owner occupied special purpose
Recorded Investment
With no related allowance recorded	1,154	1,798
With an allowance recorded	609
Unpaid Principal Balance
With no related allowance recorded	1,240	1,941
With an allowance recorded	687
Related Allowance
With an allowance recorded	113
Real estate - commercial \| Non-owner occupied general purpose
Recorded Investment
With no related allowance recorded	750	4,831
With an allowance recorded		76
Unpaid Principal Balance
With no related allowance recorded	1,160	5,653
With an allowance recorded		76
Related Allowance
With an allowance recorded		21
Real estate - commercial \| Non-owner occupied special purpose
Recorded Investment
With no related allowance recorded		1,423
With an allowance recorded	1,380
Unpaid Principal Balance
With no related allowance recorded		1,930
With an allowance recorded	1,806
Related Allowance
With an allowance recorded	163
Real estate - commercial \| Farm
Recorded Investment
With no related allowance recorded	1,370
Unpaid Principal Balance
With no related allowance recorded	1,370
Real estate - construction \| Homebuilder
Recorded Investment
With no related allowance recorded	1,320	1,791
Unpaid Principal Balance
With no related allowance recorded	1,320	1,791
Real estate - construction \| All other
Recorded Investment
With no related allowance recorded	240	291
With an allowance recorded	261	270
Unpaid Principal Balance
With no related allowance recorded	324	323
With an allowance recorded	300	306
Related Allowance
With an allowance recorded	60	98
Real estate - residential \| Investor
Recorded Investment
With no related allowance recorded	1,760	2,595
With an allowance recorded	131	135
Unpaid Principal Balance
With no related allowance recorded	2,101	3,024
With an allowance recorded	144	145
Related Allowance
With an allowance recorded	21	24
Real estate - residential \| Owner occupied
Recorded Investment
With no related allowance recorded	11,602	11,419
With an allowance recorded	152	23
Unpaid Principal Balance
With no related allowance recorded	13,113	12,816
With an allowance recorded	194	65
Related Allowance
With an allowance recorded	30	38
Real estate - residential \| Revolving and junior liens
Recorded Investment
With no related allowance recorded	2,330	2,238
With an allowance recorded	366	371
Unpaid Principal Balance
With no related allowance recorded	3,628	3,541
With an allowance recorded	405	405
Related Allowance
With an allowance recorded	$ 93	$ 97

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

Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 14A

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953550-111524

+ Details

Name:	us-gaap_ImpairedFinancingReceivableRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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

- Definition

Amount of allowance for credit losses related to recorded investment.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 15

-Subparagraph (a)(3)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5212-111524

+ Details

Name:	us-gaap_ImpairedFinancingReceivableRelatedAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of unpaid principal balance of impaired financing receivables with related allowance for credit losses and without related allowance for credit losses.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 15

-Subparagraph (a)(4)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5212-111524

+ Details

Name:	us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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

- Definition

Amount of investment in impaired financing receivables for which there is no related allowance for credit losses.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of unpaid principal balance of impaired financing receivables for which there is no related allowance for credit losses.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 15

-Subparagraph (a)(4)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5212-111524

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceUnpaidPrincipalBalance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in impaired financing receivables for which there is a related allowance for credit losses.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Unpaid principal balance of impaired financing receivables for which there is a related allowance for credit losses.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 15

-Subparagraph (a)(4)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5212-111524

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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

- Details

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

- 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: