HMS Onslow was an O-class destroyer of the Royal Navy. The O-class were intermediate destroyers, designed before the outbreak of the Second World War to meet likely demands for large number of destroyers. They had a main gun armament of four 4.7 in guns, and had a design speed of 36 kn. Onslow was ordered on 2 October 1939 and was built by John Brown & Company at their Clydebank, Glasgow shipyard, launching on 31 March 1941 and completing on 8 October 1941.

The O and P class was a class of destroyers of the British Royal Navy. Ordered in 1939, they were the first ships in the War Emergency Programme, also known as the 1st and 2nd Emergency Flotilla, respectively. They served as convoy escorts in World War II, and some were subsequently converted to fast second-rate anti-submarine frigates in the 1950s.