An oil replacement apparatus has a housing, an oil replenishing assembly and a controlling assembly. The oil replenishing assembly has a base, a vacuum pump, an oil tank and multiple control cylinders. The base has a canal set formed inside the base. The vacuum pump and the oil tank are mounted on the base and communicate with the canal set. The control cylinders are mounted on the base to close the canal set and can selectively open or close holes of the canal set. The controlling assembly is connected with the oil replenishing assembly. Accordingly, the oil replacement apparatus can automatically drain or inject oil in a hydraulic brake assembly.