An oil separator is provided with a case that has an air inlet and a plurality of expansion chambers formed within the case. Air that contains oil is introduced into the case via the inlet and caused to strike a impingement plate to thereby separate the oil from the introduced air and recover the oil. The transverse cross-sectional area of each expansion chamber is greater than the open area of the inlet. Partition walls with orifice holes formed therein are provided between the expansion chambers.