Abstract: Some engine applications place spatial constraints on heat exchangers, such as oil coolers. In such a case, a heat exchanger having an oblong rather than a circular cross section can satisfy spatial constraints, and with certain internal geometry, may permit improved heat rejection performance while potentially enabling a cost reduction through a reduction in materials. The heat exchanger has a housing which defines a heat exchanging cavity within which a tube bundle is positioned. The tube bundle is made up of a plurality of tubes in a hexagonal packing pattern that are supported by a plurality of baffles. The tube bundle and the housing define a serpentine flow path between an inlet and an outlet. The serpentine flow path includes a plurality of segments that are generally perpendicular to the tubes, and these segments are separated by flow direction changing windows.