Represents the number of real estate investments sold by the entity during the period.

No definition available.

Name:	olp_NumberOfRealEstatePropertiesSold
Namespace Prefix:	olp_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

The difference between the carrying value and the sale price of real estate or properties that were intended to be sold or held for capital appreciation or rental income. This element refers to the gain (loss) included in earnings and not to the cash proceeds of the sale. This element is a noncash adjustment to net income when calculating net cash generated by operating activities using the indirect method.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585

+ Details

Name:	us-gaap_GainLossOnSaleOfProperties
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

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

- Definition

Carrying amount as of the balance sheet date of investments in land and buildings held for sale, excluding real estate considered to be inventory of the entity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1(f))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

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

- Definition

Revenue from the sale of commercial, industrial, or residential property during the period. This element is more likely than not relevant to an entity for which real estate operations are a principal activity. If real estate operations are not a principal activity, the reporting entity would likely use a gain (loss) on sale of property, plant, or equipment type element.

+ References

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=63488584&loc=d3e20235-122688

+ Details

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

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

Name:	us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis=olp_RealEstatePropertyLocatedInKilleenTXMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis=olp_RealEstatePropertyLocatedInSandySpringsGMember
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

Name:	us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis=olp_RealEstatePropertyLocatedInTomlinsonPMember
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

Name:	us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis=olp_PropertyLocatedInMissouriMember
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

Name:	us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis=olp_PropertyLocatedInIllinoisMember
Namespace Prefix:
Data Type:	na
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Period Type:

Sale of Properties and Properties Held-for-Sale (Details)	1 Months Ended	3 Months Ended	6 Months Ended		9 Months Ended
	Jun. 30, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($) property	Sep. 30, 2017 USD ($)
Sale of Properties
Gross Sales Price				$ 40,456,000
Gain on Sales of Real Estate, Net				$ 9,705,000
Properties Held-for-Sale
Net book value of properties held for sale	$ 11,652,000		$ 11,652,000
Retail property, Greenwood Village, Colorado
Sale of Properties
Gross Sales Price			9,500,000
Gain on Sales of Real Estate, Net			6,568,000
Portfolio of eight retail properties, Louisiana and Mississippi
Sale of Properties
Number of real estate properties sold \| property				8
Gross Sales Price				$ 13,750,000
Gain on Sales of Real Estate, Net				787,000
Retail property, Killeen, TX
Sale of Properties
Gross Sales Price				3,100,000
Gain on Sales of Real Estate, Net				980,000
Land, Sandy Springs, Georgia
Sale of Properties
Gross Sales Price				8,806,000
Gain on Sales of Real Estate, Net				2,278,000
Industrial property, Tomlinson, Pennsylvania
Sale of Properties
Gross Sales Price				14,800,000
Gain on Sales of Real Estate, Net				$ 5,660,000
Property located in Missouri
Sale of Properties
Gross Sales Price	10,250,000
Gain on Sales of Real Estate, Net		$ 2,181,000			$ 2,181,000
Properties Held-for-Sale
Net book value of properties held for sale	7,909,000		7,909,000
Property located in Illinois
Sale of Properties
Gross Sales Price	5,000,000
Properties Held-for-Sale
Net book value of properties held for sale	$ 3,743,000		$ 3,743,000