The present invention relates to an oil damper for preventing a seat and a cover from rapidly rotating. The oil damper includes a shaft unit, a cam, a valve, an elastic member, a housing and a fastening member. The shaft unit has a coupling part which is coupled to the seat and the cover. The cam is rotatably coupled to a rotating shaft of the shaft unit. The valve opens or closes a coupling hole depending on movement of the cam to allow fluid to flow through the coupling hole or prevent the fluid from flowing. The elastic member is provided in the housing and is compressed by the cam. A flow rate control groove is formed in the housing and controls the flow rate of fluid in response to movement of the cam. The fastening member is fitted over the shaft unit and fastened to the housing.