An air cooled oil cooler has an upper plate, a lower plate and a plurality of tubes and outer fins disposed therebetween. Each tube contains an inner offset fin, and the outer fins formed in a corrugated shape and each having one return louver on an intermediate portion between a top portion and a bottom portion of the outer fin. The outer fins is disposed between the tubes so that the tubes and the outer fins are arranged alternatively and stacked in a pile between the upper and lower plates. The tubes are formed to be flat tubes having a height-width ratio of the tube to be 4.8-7.4%.