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OTHER INCOME ITEMS - Lease Termination Fees (Details) - USD ($) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2015 | Mar. 31, 2015 | Dec. 31, 2015 | Dec. 31, 2014
OTHER INCOME ITEMS |  |  |  |
Lease termination fee received from an industrial tenants in a lease buy-out transaction | $ 2,236,000 | $ 650,000 | $ 2,886,000 | $ 1,269,000
Write-off of entire balance of the unbilled rent receivable and the intangible lease asset |  |  | $ 530,000 | $ 150,000

X
- Definition Amount represents gross lease termination fee. + References No definition available. + Details
Name: | olp_LeaseTerminationFeeGross
Namespace Prefix: | olp_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Write-off of Unbilled Rent Receivable and Intangible Lease Asset + References No definition available. + Details
Name: | olp_WriteoffOfUnbilledRentReceivableAndIntangibleLeaseAsset
Namespace Prefix: | olp_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- References No definition available. + Details
Name: | us-gaap_OtherIncomeAndExpensesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration