Name:	osbc_ForeclosedRealEstateExpenseNetOfLeaseRevenue
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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

The net gain (loss) before provision for valuation allowances resulting from sales and other disposals of other real estate owned.

No definition available.

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Namespace Prefix:	osbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

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

Amount of operating expenses recognized during the period for real estate assets acquired through settlement or foreclosure.

No definition available.

Name:	osbc_RealEstateAcquiredThroughForeclosureOperatingExpenses
Namespace Prefix:	osbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

The amount of real estate sold during the period net of gains (losses) on sale.

No definition available.

Name:	osbc_RealEstateCostOfRealEstateSoldNetOfGainLoss
Namespace Prefix:	osbc_
Data Type:	xbrli:monetaryItemType
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Period Type:	duration

Amount of the increase in the real estate owned valuation allowance, net of miscellaneous adjustments, that is charged against earnings in the period.

No definition available.

Name:	osbc_RealEstateOwnedValuationAllowanceProvisionNetOfAdjustment
Namespace Prefix:	osbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Other real estate revenue not otherwise specified in the taxonomy.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.1(e))

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

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Period Type:	duration

- Details

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

Carrying amount as of the balance sheet date of land and buildings obtained through foreclosure proceedings or defeasance in full or partial satisfaction of a debt arrangement.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 45

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=28367877&loc=d3e4437-111522

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 28

-Article 12

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph c

-Subparagraph Schedule III

-Article 5

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

Amount of real estate investments acquired through foreclosure for entities with a substantial portion of business acquiring and holding investment real estate.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 205

-SubTopic 10

-Section S99

-Paragraph 6

-Subparagraph (SX 210.5-04.(c) Schedule III)

-URI http://asc.fasb.org/extlink&oid=27047687&loc=d3e5864-122674

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 28

-Article 12

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph c

-Subparagraph Schedule III

-Article 5

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 970

-SubTopic 360

-Section S99

-Paragraph 1

-Subparagraph (SX 210.12-28.4)

-URI http://asc.fasb.org/extlink&oid=6590653&loc=d3e638233-123024

Name:	us-gaap_RealEstateAcquisitionsThroughForeclosures
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Data Type:	xbrli:monetaryItemType
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Period Type:	duration

Amount of other deductions to real estate investments, excluding sales, for entities with a substantial portion of business acquiring and holding investment real estate.

Name:	us-gaap_RealEstateOtherDeductions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
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Period Type:	duration

For each period for which an income statement is required, disclosure of the changes in the allowance, including balances at end of period.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.10(4))

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 10

-Subparagraph 4

-Article 9

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

Amount of the valuation allowance for real estate owned that was applied in the period against the carrying amount of real estate owned that was sold, written down or written off.

Name:	us-gaap_RealEstateOwnedValuationAllowanceAmountsApplied
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
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Period Type:	duration

Amount of expense recognized in the income statement for a decrease in valuation of real estate owned (REO).