An oil separator includes a separation part and an oil storage part. The separation part includes a separation cylinder, an inner cylinder disposed in the separation cylinder and an inlet pipe connecting to the separation cylinder tangential to an inner surface of the separation cylinder. The separation part defines an inlet opening to introduce CO2 refrigerant containing oil in the separation cylinder. The separation cylinder separates the oil from the CO2 refrigerant by means of centrifugal force. A ratio of a flow rate (kg/h) of the CO2 refrigerant flowing in the separation cylinder to an area (mm2) of the inlet opening is at least 4. Alternative to or in addition to the above, a ratio of a distance (mm) from an end of the inner cylinder to a bottom wall of the separation cylinder to an inner diameter (mm) of the separation cylinder is at least 2.5.