REAL ESTATE PROPERTY, NET (Estimated Depreciation Expenses) (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Real Estate Properties Estimated Depreciation Expenses [Abstract]
2018	$ 4,204
2019	4,202
2020	4,173
2021 and thereafter	168,617
Estimated amortization expenses	$ 181,196

Amount of depreciation expenses for real estate properties, expected to be recognized during the fourth fiscal year and thereafter following the latest fiscal year.

No definition available.

Name:	obas_RealEstatePropertiesEstimatedDepreciationExpensesYearFourAndThereafter
Namespace Prefix:	obas_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant