An oil cooler for a vehicle may include a header tank partitioned by a diaphragm formed at an inside of a center in a length direction thereof, and adapted to take in a working fluid at a first side and to take out the working fluid at a second side with respect to the diaphragm, a connection tank disposed to be spaced apart from the header tank by a predetermined interval, a plurality of tubes mounted along a length direction at an interior surface of the header tank to connect with the connection tank such that the working fluid flows therethrough, and a bypass valve integrally mounted at an outside of the header tank and connected to the inside of the header tank so as to bypass or flow the working fluid flowing therein into the inflow tank by selectively opening/closing according to a temperature of the working fluid.