The invention provides an online oil-changing apparatus and system for an oil-immersed transformer, and relates to the electric power technical field. The online oil-changing apparatus comprises a first vacuum pump, a second vacuum pump, a new oil tank, a used oil tank, an inert gas tank, a four-way connector and a tee-joint connector, wherein the tee-joint connector is connected with an exhausting pipe, a second air chamber and the second vacuum pump of the oil-immersed transformer separately through a pipeline; a third air valve is arranged between the second vacuum pump and the tee-joint connector; a fourth air valve is arranged between the second air chamber and the tee-joint connector; a fifth air valve is arranged between the air exhausting pipe and the tee-joint connector; and the used oil tank is connected with an oil exhausting pipe of the oil-immersed transformer. By adoption of the online oil-changing apparatus and system, physical isolation of new oil and used oil can be realized; and it is ensured that the assembly and the newly-added transformer oil in the transformer oil tank are not in contact with air during online oil-changing of the oil-immersed transformer, so that oxidization to the internal assembly and the transformer oil of the transformer is avoided.