Abstract: An oil separator for separating liquid oil from a gaseous refrigerant in a refrigerant system has an oil separator chamber and an oil reservoir chamber arranged horizontally adjacent to one another. The oil separator chamber contains an oil separator having an inlet for receiving a refrigerant/oil mixture, a separator for separating the gas and oil from the oil/gas mixture, a gas outlet and an oil outlet leading from the very bottom of the oil separator chamber into the oil reservoir chamber. The oil reservoir chamber contains an oil return inlet opening near a bottom of the oil reservoir chamber leading to an outside of the oil reservoir chamber. Also, an arrangement is provided at the oil outlet for allowing a flow of oil from the oil separator chamber to the oil reservoir chamber, but preventing a flow of oil from the oil reservoir chamber to the oil separator chamber.