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Commitments (Details) - Building located In Hauppauge, New York - USD ($) | Dec. 31, 2018 | Jun. 30, 2018
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Contractual obligation | $ 7,800,000 |
Amount spent to date for improvement obligation |  | $ 6,261,000

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- 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

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