An outdoor unit of the air conditioner includes a housing including a base, a refrigerant pipe through which a refrigerant flows, a water pipe through which water flows, a plate heat exchanger disposed inside the housing to exchange heat between water flowing along the water pipe and the refrigerant flowing along the refrigerant pipe, an expansion tank disposed inside the housing connected to the water pipe and configured to regulate a pressure in the water pipe, a control box disposed inside the housing and configured to accommodate a printed circuit board, a first bracket fixing the plate heat exchanger to the base, a second bracket fixing the expansion tank to the base and be fastened to the first bracket, and a third bracket supporting the control box and fastened to an upper surface of the second bracket.