An oil separator is provided with a casing having an inlet for air and an outlet for air, an expansion chamber for expanding air introduced therein through the inlet, an accommodation member communicating with the expansion chamber in the vertical direction, a collected liquid storage portion provided below the accommodation member, and a connecting hose having an oil separator connection end for being connected to the inlet and an air dryer connection end for being connected to an air dryer. The connecting hose is configured such that the oil separator connection end is located higher than the air dryer connection end.