An oil separator, comprising: an inlet configured to receive a refrigerant; a duct through which, during operation of the oil separator, the refrigerant flows, wherein the duct and the inlet each have an axis in a same direction, and the refrigerant, during operation of the oil separator, enters the oil separator along the axis; and an oil separation assembly through which, during operation of the oil separator, the refrigerant passing through the duct passes, the duct having a circumferential wall and comprising: a first section comprising a first muffler adjacent to the inlet, the first muffler comprising a chamber defined by a first wall of the first section and designed to have a length dimension in a length direction of the duct and a depth dimension intersecting the length dimension, wherein the chamber comprises: a separation plate; a first space, which is provided with the separation plate in a direction of a flow path of the refrigerant and having a first length dimension; a second space downstream of the first space, the second space having a second length dimension; and a connection passage between the first space and the second space, via which, during operation of the oil separator, the refrigerant flows from the first space to the second space; and a second section downstream of the first section, a second circumferential wall of the second section being provided with a second muffler; wherein a distance from the first wall to the axis is greater than a distance from the second wall to the axis. an inlet configured to receive a refrigerant; a duct through which, during operation of the oil separator, the refrigerant flows, wherein the duct and the inlet each have an axis in a same direction, and the refrigerant, during operation of the oil separator, enters the oil separator along the axis; and an oil separation assembly through which, during operation of the oil separator, the refrigerant passing through the duct passes, the duct having a circumferential wall and comprising: a first section comprising a first muffler adjacent to the inlet, the first muffler comprising a chamber defined by a first wall of the first section and designed to have a length dimension in a length direction of the duct and a depth dimension intersecting the length dimension, wherein the chamber comprises: a separation plate; a first space, which is provided with the separation plate in a direction of a flow path of the refrigerant and having a first length dimension; a second space downstream of the first space, the second space having a second length dimension; and a connection passage between the first space and the second space, via which, during operation of the oil separator, the refrigerant flows from the first space to the second space; and a second section downstream of the first section, a second circumferential wall of the second section being provided with a second muffler; wherein a distance from the first wall to the axis is greater than a distance from the second wall to the axis. a separation plate; a first space, which is provided with the separation plate in a direction of a flow path of the refrigerant and having a first length dimension; a second space downstream of the first space, the second space having a second length dimension; and a connection passage between the first space and the second space, via which, during operation of the oil separator, the refrigerant flows from the first space to the second space; and a second section downstream of the first section, a second circumferential wall of the second section being provided with a second muffler; wherein a distance from the first wall to the axis is greater than a distance from the second wall to the axis.