Loans - Impaired Loans (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Recorded Investment
With no related allowance recorded	$ 39,197		$ 35,075
With an allowance recorded	1,589		11,495
Total impaired loans	40,786		46,570
Unpaid Principal Balance
With no related allowance recorded	46,512		42,799
With an allowance recorded	1,889		13,996
Total impaired loans	48,401		56,795
Related Allowance
With an allowance recorded	541		2,395
Average Recorded Investment
With no related allowance recorded	37,136	53,456
With an allowance recorded	6,542	18,162
Total impaired loans	43,678	71,618
Interest Income Recognized
With no related allowance recorded	359	392
With an allowance recorded	15	12
Total impaired loans	374	404
Commercial
Recorded Investment
With no related allowance recorded	1,645		27
Unpaid Principal Balance
With no related allowance recorded	2,173		34
Average Recorded Investment
With no related allowance recorded	836	113
With an allowance recorded		283
Real estate - commercial \| Owner occupied general purpose
Recorded Investment
With no related allowance recorded	7,176		2,543
With an allowance recorded			730
Unpaid Principal Balance
With no related allowance recorded	7,840		3,006
With an allowance recorded			792
Related Allowance
With an allowance recorded			264
Average Recorded Investment
With no related allowance recorded	4,860	3,565
With an allowance recorded	365	974
Interest Income Recognized
With no related allowance recorded	81	3
Real estate - commercial \| Owner occupied special purpose
Recorded Investment
With no related allowance recorded	3,217		3,371
With an allowance recorded	585		4,300
Unpaid Principal Balance
With no related allowance recorded	4,147		4,117
With an allowance recorded	676		4,702
Related Allowance
With an allowance recorded	167		759
Average Recorded Investment
With no related allowance recorded	3,294	5,913
With an allowance recorded	2,443	2,777
Real estate - commercial \| Non-owner occupied general purpose
Recorded Investment
With no related allowance recorded	7,065		5,428
With an allowance recorded			939
Unpaid Principal Balance
With no related allowance recorded	8,052		6,709
With an allowance recorded			1,030
Related Allowance
With an allowance recorded			129
Average Recorded Investment
With no related allowance recorded	6,246	11,995
With an allowance recorded	469	1,481
Interest Income Recognized
With no related allowance recorded	45	113
Real estate - commercial \| Non-owner occupied special purpose
Recorded Investment
With no related allowance recorded	1,472		661
Unpaid Principal Balance
With no related allowance recorded	1,935		919
Average Recorded Investment
With no related allowance recorded	1,067	457
Real estate - commercial \| Retail properties
Recorded Investment
With no related allowance recorded			3,144
Unpaid Principal Balance
With no related allowance recorded			3,811
Average Recorded Investment
With no related allowance recorded	1,572	6,918
With an allowance recorded		876
Real estate - commercial \| Farm
Average Recorded Investment
With no related allowance recorded		1,259
Real estate - construction \| Homebuilder
Recorded Investment
With no related allowance recorded	1,791		2,016
With an allowance recorded			168
Unpaid Principal Balance
With no related allowance recorded	1,791		2,016
With an allowance recorded			604
Related Allowance
With an allowance recorded			76
Average Recorded Investment
With no related allowance recorded	1,903	3,597
With an allowance recorded	84	97
Interest Income Recognized
With no related allowance recorded	69	120
Real estate - construction \| Land
Recorded Investment
With no related allowance recorded	209		209
Unpaid Principal Balance
With no related allowance recorded	312		308
Average Recorded Investment
With no related allowance recorded	209	231
Real estate - construction \| Commercial speculative
Recorded Investment
With no related allowance recorded			738
With an allowance recorded			1,175
Unpaid Principal Balance
With no related allowance recorded			742
With an allowance recorded			1,808
Related Allowance
With an allowance recorded			17
Average Recorded Investment
With no related allowance recorded	369	2,089
With an allowance recorded	588	2,971
Real estate - construction \| All other
Recorded Investment
With no related allowance recorded	306		4
With an allowance recorded	279		436
Unpaid Principal Balance
With no related allowance recorded	347		35
With an allowance recorded	312		468
Related Allowance
With an allowance recorded	107		262
Average Recorded Investment
With no related allowance recorded	155	188
With an allowance recorded	357	465
Real estate - residential \| Investor
Recorded Investment
With no related allowance recorded	2,553		5,984
With an allowance recorded	139		684
Unpaid Principal Balance
With no related allowance recorded	3,538		8,338
With an allowance recorded	149		913
Related Allowance
With an allowance recorded	79		160
Average Recorded Investment
With no related allowance recorded	4,269	5,845
With an allowance recorded	412	3,263
Interest Income Recognized
With no related allowance recorded	32
Real estate - residential \| Owner occupied
Recorded Investment
With no related allowance recorded	11,735		9,179
With an allowance recorded	259		1,565
Unpaid Principal Balance
With no related allowance recorded	13,214		10,451
With an allowance recorded	394		1,831
Related Allowance
With an allowance recorded	59		170
Average Recorded Investment
With no related allowance recorded	10,457	9,606
With an allowance recorded	912	3,448
Interest Income Recognized
With no related allowance recorded	128	151
With an allowance recorded	15	12
Real estate - residential \| Revolving and junior liens
Recorded Investment
With no related allowance recorded	2,028		1,771
With an allowance recorded	327		1,498
Unpaid Principal Balance
With no related allowance recorded	3,163		2,313
With an allowance recorded	358		1,848
Related Allowance
With an allowance recorded	129		558
Average Recorded Investment
With no related allowance recorded	1,899	1,668
With an allowance recorded	912	1,527
Interest Income Recognized
With no related allowance recorded	4	5
Consumer
Average Recorded Investment
With no related allowance recorded		$ 12

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

Average 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 15

-Subparagraph (c)(1)

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

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 17

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5258-111524

+ Details

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

- Details

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

- Definition

Amount of interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

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

- Definition

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

+ 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

+ 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 average investment of impaired financing receivables for which there is no related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount of interest income, recognized on accrual basis, on financing receivables that are recognized as impaired, for which there is no related allowance for credit losses.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceInterestIncomeAccrualMethod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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

+ Details

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

- Definition

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

+ References

+ Details

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

- Definition

Amount of interest income, recognized on accrual basis, on financing receivables, for which there is a related allowance for credit losses and are recognized as impaired.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceInterestIncomeAccrualMethod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- 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

+ Details

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