Abstract: A compact yet efficient auxiliary transmission oil cooler of simple design and adapted to be installed within the engine coolant hose of a vehicle. The oil cooler consists of two basic parts. The first part is an outer tubular sleeve that serves as a housing and has opposite open ends adapted to be attached at its ends to a radiator hose for passage of coolant through the tube. The second part is the oil passage means to accomplish heat transfer to the coolant and it consists of a tubular member having a plurality of generally parallel runs or passes extending inside the coolant hose.