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) comprising a case and a lid, the lid including an inlet for introducing the purge air from the air dryer and an outlet for discharging cleaned air; a first expansion chamber and a second expansion chamber (45, 51; 345, 351) arranged next to one another in a vertical direction, wherein the inlet is provided on a side of the lid that faces in a horizontal direction, wherein the internal space of the lid functions as the first expansion chamber, a collected liquid storage portion (54; 254; 354) for storing the collected liquid; and a drain outlet (33; 333) for draining the collected liquid, wherein the oil separator, in use, introduces the purge air into the housing through the inlet, and causes the purge air to strike an impingement member to separate oil from the introduced purge air, thereby recovering the oil, and discharges cleaned air from the outlet; wherein a baffle plate (46; 346) is provided in the housing at a position in front of the inlet, the baffle plate obstructing the introduced purge air from flowing straight ahead. 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) comprising a case and a lid, the lid including an inlet for introducing the purge air from the air dryer and an outlet for discharging cleaned air; a first expansion chamber and a second expansion chamber (45, 51; 345, 351) arranged next to one another in a vertical direction, wherein the inlet is provided on a side of the lid that faces in a horizontal direction, wherein the internal space of the lid functions as the first expansion chamber, a collected liquid storage portion (54; 254; 354) for storing the collected liquid; and a drain outlet (33; 333) for draining the collected liquid, wherein the oil separator, in use, introduces the purge air into the housing through the inlet, and causes the purge air to strike an impingement member to separate oil from the introduced purge air, thereby recovering the oil, and discharges cleaned air from the outlet; wherein a baffle plate (46; 346) is provided in the housing at a position in front of the inlet, the baffle plate obstructing the introduced purge air from flowing straight ahead. a housing (31, 32; 231; 331, 332) comprising a case and a lid, the lid including an inlet for introducing the purge air from the air dryer and an outlet for discharging cleaned air; a first expansion chamber and a second expansion chamber (45, 51; 345, 351) arranged next to one another in a vertical direction, wherein the inlet is provided on a side of the lid that faces in a horizontal direction, wherein the internal space of the lid functions as the first expansion chamber, a collected liquid storage portion (54; 254; 354) for storing the collected liquid; and a drain outlet (33; 333) for draining the collected liquid, wherein the oil separator, in use, introduces the purge air into the housing through the inlet, and causes the purge air to strike an impingement member to separate oil from the introduced purge air, thereby recovering the oil, and discharges cleaned air from the outlet; wherein a baffle plate (46; 346) is provided in the housing at a position in front of the inlet, the baffle plate obstructing the introduced purge air from flowing straight ahead.