Fair Value Measurements - Quantitative and Qualitative Information (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2015	Dec. 31, 2014
Quantitative information about Level 3 fair value measurements
Mortgage servicing rights, fair value	$ 5,470	$ 5,462
Recurring
Quantitative information about Level 3 fair value measurements
Mortgage servicing rights, fair value	5,470	5,462
Recurring \| Level 3
Quantitative information about Level 3 fair value measurements
Mortgage servicing rights, fair value	5,470	5,462
Recurring \| Mortgage servicing rights \| Discounted Cash Flow \| Level 3
Quantitative information about Level 3 fair value measurements
Mortgage servicing rights, fair value	$ 5,470	$ 5,462
Recurring \| Mortgage servicing rights \| Discounted Cash Flow \| Level 3 \| Minimum
Quantitative information about Level 3 fair value measurements
Discount rate (as a percent)	9.70%	9.70%
Prepayment speed (as a percent)	6.00%	5.00%
Recurring \| Mortgage servicing rights \| Discounted Cash Flow \| Level 3 \| Maximum
Quantitative information about Level 3 fair value measurements
Discount rate (as a percent)	15.50%	108.20%
Prepayment speed (as a percent)	34.20%	78.40%
Recurring \| Mortgage servicing rights \| Discounted Cash Flow \| Level 3 \| Weighted Average of Inputs
Quantitative information about Level 3 fair value measurements
Discount rate (as a percent)	10.20%	10.20%
Prepayment speed (as a percent)	11.00%	10.90%
Recurring \| Asset-backed securities \| Discounted Cash Flow \| Level 3
Quantitative information about Level 3 fair value measurements
Asset-backed securities		$ 52,941
Recurring \| Asset-backed securities \| Discounted Cash Flow \| Level 3 \| Minimum
Quantitative information about Level 3 fair value measurements
Credit risk premium (as a percent)		0.90%
Liquidity Discount (as a percent)		3.50%
Recurring \| Asset-backed securities \| Discounted Cash Flow \| Level 3 \| Maximum
Quantitative information about Level 3 fair value measurements
Credit risk premium (as a percent)		0.90%
Liquidity Discount (as a percent)		3.70%
Recurring \| Asset-backed securities \| Discounted Cash Flow \| Level 3 \| Weighted Average of Inputs
Quantitative information about Level 3 fair value measurements
Credit risk premium (as a percent)		0.90%
Liquidity Discount (as a percent)		3.60%

Represents the credit risk premium expressed as a percentage, used as an input to measure fair value.

No definition available.

Name:	osbc_FairValueInputsCreditRiskPremium
Namespace Prefix:	osbc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the liquidity discount expressed as a percentage, used as an input to measure fair value.

+ References

No definition available.

+ Details

Name:	osbc_FairValueInputsLiquidityDiscount
Namespace Prefix:	osbc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Investments in notes or bonds that are collateralized by a specific group of underlying assets owned by the issuer of the notes or bonds which are short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Cash Equivalents

-URI http://asc.fasb.org/extlink&oid=6507016

+ Details

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

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

- Definition

Interest rate used to find the present value of an amount to be paid or received in the future as an input to measure fair value. For example, but not limited to, weighted average cost of capital (WACC), cost of capital, cost of equity and cost of debt.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (bbb)

-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueInputsDiscountRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Rate at which loans or a loan portfolio are expected to prepay principal balances, used as an input to measure fair value.

+ References

+ Details

Name:	us-gaap_FairValueInputsPrepaymentRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Fair value of an asset representing net future revenue from contractually specified servicing fees, late charges, and other ancillary revenues, in excess of future costs related to servicing arrangements.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 860

-SubTopic 50

-Section 50

-Paragraph 3

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=7882072&loc=d3e122625-111746

+ Details

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

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=osbc_MortgageServicingRightsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ValuationTechniqueAxis=osbc_DiscountedCashFlowValuationTechniqueMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_WeightedAverageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=us-gaap_AssetBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: