Abstract: An oil separator device for removing a substantial portion of the oil contained in the crankcase fumes of an internal combustion engine. The device is designed to be mounted on the inside of a rocker arm cover of the engine as a part of the crankcase ventilation system. Internal baffles direct a laminar flow of crankcase fumes through the separator canister along a folded path from an inlet in the bottom of the canister to an outlet in the top. Partially isolated channels extend along opposite edges of the flow path. Oil in the fumes attaches itself to the internal surfaces of the separator and flows down these surfaces, which are inclined, to a sump in the bottom. A tubular assembly connected to the bottom of the separator provides combined fume inlet and oil return passageways.