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COMMITMENTS AND CONTINGENCIES (Details) | 12 Months Ended
--- | ---
Dec. 31, 2018 USD ($) item | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Commitments |  |  |
Pension expense | $ 295,000 | $ 275,000 | $ 273,000
Annual fixed leasehold rent through July 2019 | 371,094 |  |
Annual fixed leasehold rent thereafter through March 3, 2020 | $ 463,867 |  |
Number of 5-year extension options | item | 5 |  |
Lease extension period for the first five renewal options | 5 years |  |
Number of seven month extension options | item | 1 |  |
Lease extension period for the last renewal option | 7 months |  |
Building Located In Hauppauge New York | Building expansion and improvements |  |  |
Commitments |  |  |
Contractual obligation | $ 791,000 |  |

X
- Definition Represents the amount of annual fixed rent required to be paid by the entity thereafter through March 3, 2020. + References No definition available. + Details
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Namespace Prefix: | olp_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of lessee's undiscounted obligation for annual lease payments for operating lease through July 2019. + References No definition available. + Details
Name: | olp_OperatingLeasesAnnualFixedRentThroughJuly2019
Namespace Prefix: | olp_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Represents the period for which the lease may be extended under the first five renewal options. + References No definition available. + Details
Name: | olp_OperatingLeasesExtensionPeriodFirstFiveRenewalOptions
Namespace Prefix: | olp_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the period for which the lease may be extended under the last renewal option. + References No definition available. + Details
Name: | olp_OperatingLeasesExtensionPeriodLastRenewalOption
Namespace Prefix: | olp_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the number of five year extension options available to entity. + References No definition available. + Details
Name: | olp_OperatingLeasesNumberOfFiveYearExtensionOptions
Namespace Prefix: | olp_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Represents the number of seven months extension options available to entity. + References No definition available. + Details
Name: | olp_OperatingLeasesNumberOfSevenMonthsExtensionOptions
Namespace Prefix: | olp_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- 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
- 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_OtherCommitmentsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of cost (reversal of cost) for pension benefits. Excludes other postretirement benefits. + References No definition available. + Details
Name: | us-gaap_PensionExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | srt_RealEstateAndAccumulatedDepreciationDescriptionOfPropertyAxis=olp_BuildingLocatedInHauppaugeNewYorkMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_BuildingAndBuildingImprovementsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |