 "Onslow" had a main gun armament of four 45-calibre 4.7-inch (120 mm) Mark IX guns in single mounts.The ship was designed to carry two quadruple 21-inch (533 mm) torpedo tubes, but early experience of the vulnerability of destroyers to air attack off Norway and during the evacuation from Dunkirk in 1940 resulted in the armament of the O-class being revised during construction, with the aft set of torpedo-tubes removed and replaced by a single 4 in (102 mm) QF Mark V anti-aircraft gun, although the 4-inch gun was later removed and the second bank of torpedo tubes re-instated.