An oil separator is provided with a housing having an inlet for air, and an expansion chamber located inside the housing. The oil separator introduces air containing oil that has passed through the inlet into the housing, and separates and recovers the oil from the introduced air. The transverse cross-sectional area of the expansion chamber is larger than the opening area of the inlet. The oil separator is further provided with a collected liquid storage portion that is located below the expansion chamber and collects the separated oil, a liquid communication portion having a communication hole that connects the expansion chamber to the collected liquid storage portion, and a mounting and dismounting mechanism for mounting and dismounting the collected liquid storage portion to and from the housing.