An oil separator is provided that efficiently recovers oil mist in blow-by gas regardless of differences in particle diameter. The oil separator includes a supply space to which blow-by gas from an internal combustion engine is supplied, and multiple cyclone-type oil separation units arranged along the gas supply direction, each having a cylindrical cylinder case part to which the blow-by gas from the supply space is supplied. An inner diameter of the cylinder case part of the oil separation unit is set larger with the oil separation unit arranged more upstream in the gas supply direction.