Abstract: A heat exchanger, especially a heat exchanger for use in a motor vehicle, has a water header which includes a hollow header cover in which an oil cooling radiator is fitted. The header cover comprises a side wall through which the two tubular connectors of the oil cooler are inserted. The header cover also has a curved wall lying opposite the side wall, so as to define an internal cavity which leaves enough space widthwise for the oil cooler to be fitted within the header cover. The latter has, in this internal cavity, an internal width which is greater than the overall width of the oil cooler, and, at the level of its open face, an internal width which is smaller than the overall width of the oil cooler.