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Commitments (Details) - USD ($) | Dec. 31, 2018 | Jun. 30, 2017
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Building located In Hauppauge, New York |  |
Contractual obligation | $ 7,800,000 | $ 436,000
Regal Cinemas In Greensboro, North Carolina |  |
Contractual obligation |  | $ 3,000,000

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

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

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