A system comprising: an air dryer configured to dry compressed air supplied from a compressor and to supply dried compressed air, the air dryer comprising a supplying line for supplying the dried compressed air and an exhaust line for discharging purge air; characterised by an oil separator connected to the exhaust line, the oil separator comprising: a housing (31, 32; 231; 331, 332) including an inlet for introducing the purge air from the air dryer and an outlet for discharging cleaned air; an expansion chamber (45, 51, 345, 351), which is located in the housing and expands the purge air that has been introduced through the inlet; an accommodation member (48; 348), which is located in the housing and communicates with the expansion chamber in the vertical direction, wherein the accommodation member accommodates an impingement member (50; 350); a communication section (32a; 332a) that connects the inside of the housing and the outlet to each other; and a collected liquid storage portion (54; 254; 354) located below the accommodation member, wherein the oil separator introduces the purge air into the housing through the inlet, and causes the purge air to strike the impingement member to separate oil from the introduced purge air, thereby recovering the oil, the inlet and the outlet are located in an upper section of the housing and provided on a side that faces in the horizontal direction, and the accommodation member separates oil from the introduced purge air and discharges the cleaned air sideways. an air dryer configured to dry compressed air supplied from a compressor and to supply dried compressed air, the air dryer comprising a supplying line for supplying the dried compressed air and an exhaust line for discharging purge air; characterised by an oil separator connected to the exhaust line, the oil separator comprising: a housing (31, 32; 231; 331, 332) including an inlet for introducing the purge air from the air dryer and an outlet for discharging cleaned air; an expansion chamber (45, 51, 345, 351), which is located in the housing and expands the purge air that has been introduced through the inlet; an accommodation member (48; 348), which is located in the housing and communicates with the expansion chamber in the vertical direction, wherein the accommodation member accommodates an impingement member (50; 350); a communication section (32a; 332a) that connects the inside of the housing and the outlet to each other; and a collected liquid storage portion (54; 254; 354) located below the accommodation member, wherein a housing (31, 32; 231; 331, 332) including an inlet for introducing the purge air from the air dryer and an outlet for discharging cleaned air; an expansion chamber (45, 51, 345, 351), which is located in the housing and expands the purge air that has been introduced through the inlet; an accommodation member (48; 348), which is located in the housing and communicates with the expansion chamber in the vertical direction, wherein the accommodation member accommodates an impingement member (50; 350); a communication section (32a; 332a) that connects the inside of the housing and the outlet to each other; and a collected liquid storage portion (54; 254; 354) located below the accommodation member, wherein the oil separator introduces the purge air into the housing through the inlet, and causes the purge air to strike the impingement member to separate oil from the introduced purge air, thereby recovering the oil, the inlet and the outlet are located in an upper section of the housing and provided on a side that faces in the horizontal direction, and the accommodation member separates oil from the introduced purge air and discharges the cleaned air sideways.