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 (123, 223) that discharges purge air containing oil to the oil separator, the oil separator comprises: a housing (4, 311) including an inlet (7, 135, 235) that is connected to the purge air outlet of the air dryer for introducing the purge air from the air dryer; an expansion chamber (31, 145, 245) provided in the housing (4, 311); wherein the oil separator being configured to introduce purge air containing oil into the housing (4, 311) through the inlet (7, 14, 135, 235), and separates and recovers the oil from the introduced purge air, and the transverse cross-sectional area of the expansion chamber (31, 145, 245) is greater than the opening area of the inlet, characterized in that the oil separator further comprises a collected liquid storage portion at the lower part of the housing, and a heating device (41, 155, 280), which heats a lower part of the expansion chamber, wherein the collected liquid storage portion stores the collected liquid containing separated oil, and the heating device is configured to heat the collected liquid in the collected liquid storage portion. 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 (123, 223) that discharges purge air containing oil to the oil separator, the oil separator comprises: a housing (4, 311) including an inlet (7, 135, 235) that is connected to the purge air outlet of the air dryer for introducing the purge air from the air dryer; an expansion chamber (31, 145, 245) provided in the housing (4, 311); wherein the oil separator being configured to introduce purge air containing oil into the housing (4, 311) through the inlet (7, 14, 135, 235), and separates and recovers the oil from the introduced purge air, and the transverse cross-sectional area of the expansion chamber (31, 145, 245) is greater than the opening area of the inlet, characterized in that the oil separator further comprises a collected liquid storage portion at the lower part of the housing, and a heating device (41, 155, 280), which heats a lower part of the expansion chamber, wherein the collected liquid storage portion stores the collected liquid containing separated oil, and the heating device is configured to heat the collected liquid in the collected liquid storage portion. a housing (4, 311) including an inlet (7, 135, 235) that is connected to the purge air outlet of the air dryer for introducing the purge air from the air dryer; an expansion chamber (31, 145, 245) provided in the housing (4, 311); wherein the oil separator being configured to introduce purge air containing oil into the housing (4, 311) through the inlet (7, 14, 135, 235), and separates and recovers the oil from the introduced purge air, and the transverse cross-sectional area of the expansion chamber (31, 145, 245) is greater than the opening area of the inlet, characterized in that the oil separator further comprises a collected liquid storage portion at the lower part of the housing, and a heating device (41, 155, 280), which heats a lower part of the expansion chamber, wherein the collected liquid storage portion stores the collected liquid containing separated oil, and the heating device is configured to heat the collected liquid in the collected liquid storage portion.