An air system comprising a compressor, an air dryer and an oil separator, wherein the air dryer comprises: an inlet that is connected to the compressor and introduces compressed air from the compressor, an outlet that discharges dried compressed air, and a purge air outlet that discharges purge air containing oil to the oil separator, and the oil separator comprises: a housing including an inlet that is connected to the purge air outlet of the air dryer and introduces the purge air from the air dryer and an outlet that discharges cleaned air that does not contain oil to the outside; and an expansion chamber provided in the housing, wherein the oil separator introduces air containing oil into the housing through the inlet to separate and recover the oil from the introduced air, the transverse cross-sectional area of the expansion chamber is greater than the opening area of the inlet, and the oil separate further includes a collected liquid storage portion located below the inlet, the outlet, and the expansion chamber, wherein the collected liquid storage portion stores the separated oil, a liquid communication portion having a communication hole that connects the expansion chamber and the collected liquid storage portion with each other, and a mounting and dismounting mechanism (21, 22) for mounting the collected liquid storage portion to and dismounting the collected liquid storage portion from the housing. an inlet that is connected to the compressor and introduces compressed air from the compressor, an outlet that discharges dried compressed air, and a purge air outlet that discharges purge air containing oil to the oil separator, and the oil separator comprises: a housing including an inlet that is connected to the purge air outlet of the air dryer and introduces the purge air from the air dryer and an outlet that discharges cleaned air that does not contain oil to the outside; and an expansion chamber provided in the housing, wherein the oil separator introduces air containing oil into the housing through the inlet to separate and recover the oil from the introduced air, the transverse cross-sectional area of the expansion chamber is greater than the opening area of the inlet, and the oil separate further includes a collected liquid storage portion located below the inlet, the outlet, and the expansion chamber, wherein the collected liquid storage portion stores the separated oil, a liquid communication portion having a communication hole that connects the expansion chamber and the collected liquid storage portion with each other, and a mounting and dismounting mechanism (21, 22) for mounting the collected liquid storage portion to and dismounting the collected liquid storage portion from the housing. a housing including an inlet that is connected to the purge air outlet of the air dryer and introduces the purge air from the air dryer and an outlet that discharges cleaned air that does not contain oil to the outside; and an expansion chamber provided in the housing, wherein the oil separator introduces air containing oil into the housing through the inlet to separate and recover the oil from the introduced air, the transverse cross-sectional area of the expansion chamber is greater than the opening area of the inlet, and the oil separate further includes a collected liquid storage portion located below the inlet, the outlet, and the expansion chamber, wherein the collected liquid storage portion stores the separated oil, a liquid communication portion having a communication hole that connects the expansion chamber and the collected liquid storage portion with each other, and a mounting and dismounting mechanism (21, 22) for mounting the collected liquid storage portion to and dismounting the collected liquid storage portion from the housing.