An outdoor unit of an air conditioner includes a base panel having first inlet and outlet holes; a compressor; a tank storing water exchanging heat with refrigerant discharged from the compressor; a pump supplying the water to the tank; a vertical panel having second inlet and outlet holes; a deaerator removing gas contained in the heat-exchanged water flowing from the tank; and pipes connected to the tank, the pump, and the deaerator, the pipes including a first pipe connected to an inlet of the pump; a second pipe connecting an outlet of the pump and the tank; a third pipe connecting that tank and an inlet of the deaerator; and a fourth pipe connected to an outlet of the deaerator; and the first pipe and the fourth being connectable to the inlet and outlet holes of either the vertical panel or the base panel.