The invention provides an oil buffer and a high-voltage direct-current breaker. The oil buffer comprises a sealing oil cylinder which extends in a front-and-back mode and a piston in sliding fit with the sealing oil cylinder in a guiding mode. The sealing oil cylinder is filled with buffering oil, and the front end of the sealing oil cylinder is provided with a buffering section in sliding fit with the outer diameter of the piston in a guiding mode. Buffering curved surfaces enabling the gaps between the buffering curved surfaces and the inner wall face of the buffering section to be gradually enlarged when the piston moves into the buffering section are arranged on the peripheral face of the piston in the contracting direction of the piston. The curved surfaces of different shapes can be manufactured on the piston of the oil buffer quite easily, manufacturing is easy, the flow of the buffering oil discharged from the gaps between the curved surfaces and the oil cylinder can be controlled by designing the different curved surfaces, buffering of the piston in different moving states can be met, and meanwhile due to the fact that other small movable parts do not exist in the sealing oil cylinder, the sealing oil cylinder is not likely to be damaged.