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Commitments (Details) - Building located In Hauppauge, New York - USD ($)	Dec. 31, 2018	Mar. 31, 2018
Contractual obligation	$ 7,800,000
Amount spent to date for improvement obligation		$ 5,256,000

X

- Definition

Amount spent on contractual obligation, including but not limited to, long-term debt, capital lease obligations, operating lease obligations, purchase obligations, and other commitments.

+ References

No definition available.

+ Details

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Namespace Prefix:	olp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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- Definition

Amount of contractual obligation, including but not limited to, long-term debt, capital lease obligations, operating lease obligations, purchase obligations, and other commitments.

+ References

No definition available.

+ Details

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

X

- Details

Name:	us-gaap_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=olp_BuildingLocatedInHauppaugeNewyorkMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: