No definition available.

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

- 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_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=66008989&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=66008989&loc=SL6953682-111524

+ Details

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

- Definition

Amount of investment related to financing receivables modified by troubled debt restructurings within the previous 12 months, for which there was a payment default.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Recorded Investment in financing receivables that are past due at the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Recorded Investment
-URI http://asc.fasb.org/extlink&oid=65880944

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7A
-URI http://asc.fasb.org/extlink&oid=66008989&loc=SL6953401-111524

+ Details

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

- References

No definition available.

+ Details

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

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=osbc_OthersMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=osbc_HomeAffordableModificationProgramMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=osbc_CommercialRealEstateOwnerOccupiedGeneralPurposeLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=osbc_CommercialRealEstateNonOwnerOccupiedGeneralPurposeLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=osbc_ResidentialInvestorLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=osbc_ResidentialOwnerOccupiedLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=osbc_ResidentialRevolvingAndJuniorLiensLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

Loans - TDR (Details)	3 Months Ended		6 Months Ended
Loans - TDR (Details)	Jun. 30, 2017 USD ($) contract	Jun. 30, 2016 USD ($) contract	Jun. 30, 2017 USD ($) contract	Jun. 30, 2016 USD ($) contract	Dec. 31, 2016 USD ($)
Troubled debt restructurings - modified during the period
# of contracts \| contract	2		6	7
Pre-modification recorded investment	$ 155,000	$ 39,000	$ 399,000	$ 1,020,000
Post-modification recorded investment	147,000	$ 39,000	388,000	895,000
TDR's defaulted
# of contracts \| contract		1
Pre-modification outstanding recorded investment during the period			0	$ 0
Other information
Total Past Due	$ 6,088,000		$ 6,088,000		$ 5,139,000
Other
Troubled debt restructurings - modified during the period
# of contracts \| contract	2		6
Pre-modification recorded investment	$ 155,000		$ 399,000
Post-modification recorded investment	147,000		388,000
HAMP
Troubled debt restructurings - modified during the period
# of contracts \| contract				4
Pre-modification recorded investment		$ 39,000		$ 469,000
Post-modification recorded investment		$ 39,000		$ 438,000
TDR's defaulted
# of contracts \| contract		1
Real estate - commercial \| Other
Troubled debt restructurings - modified during the period
# of contracts \| contract				2
Pre-modification recorded investment				$ 312,000
Post-modification recorded investment				$ 220,000
Real estate - commercial \| Owner occupied general purpose
Other information
Total Past Due	629,000		629,000		758,000
Real estate - commercial \| Non-owner occupied general purpose
Other information
Total Past Due	835,000		835,000		1,046,000
Real estate - residential \| Investor
Other information
Total Past Due	98,000		98,000		237,000
Real estate - residential \| Owner occupied
Other information
Total Past Due	279,000		279,000		274,000
Real estate - residential \| Owner occupied \| HAMP
Troubled debt restructurings - modified during the period
# of contracts \| contract				1
Pre-modification recorded investment				$ 239,000
Post-modification recorded investment				$ 237,000
Real estate - residential \| Revolving and junior liens
Other information
Total Past Due	631,000		631,000		630,000
Commercial
Other information
Total Past Due	$ 21,000		$ 21,000		$ 131,000