The O/P-classes were the first of Britain's War Emergency Programme 'utility' destroyers of which 112 were built during World War II for the Royal Navy and allied navies. They were based on the hull and machinery of the pre-war J-class and were differentiated primarily by their armament and fire control equipment which became progressively more sophisticated. The O/P-classes (also known as the 1st and 2nd Emergency Flotillas) had different armament configurations, consisting of either four single 4.7-in QF Mk. IX guns (in half of the O-class ships) or 4-in Mk. V HA guns in the remainder. Neither was a particularly effective dual-purpose gun, and highlighted the utility destroyers' main weakness: protection against air attack, which was exacerbated by their limited ability to carry more weapons (usually this came at the cost of removing a turret or some torpedo tubes in order to conserve weight). This issue was not satisfactorily resolved until the final flotillas, late in the war. Anti-aircraft armament consisted of a quadruple 2-pdr pompom mount and around 4-8 20-mm guns later in the war. Four of the O-class ships (HMS Offa, HMS Onslaught, HMS Onslow, and HMS Oribi) were fitted as destroyer-minelayers with one less turret than their sisters and up to 60 mines. Additionally, all of the ships could carry an extra 4-in gun in place of four torpedo tubes (this was standard on the O-class, but only applied temporarily on the P-class) and carried a standard complement of 70 depth charges. After the war, some of the survivors were converted to Type 16 anti-submarine frigates, while others were sold to Pakistan and Turkey.

Ships of these two classes were quite active throughout World War II, serving in the Atlantic, Mediterranean, and (a few) in the Indian and Pacific Oceans. Up to five of the O-class ships were in action during the Battle of the Barents Sea. Although none of the O-class ships were lost, five out of the eight P-class ships were sunk or irreparably damaged, mostly in the Mediterranean (HMS Pathfinder was damaged by Japanese aircraft off Burma late in the war and never repaired). HMS Porcupine had one of the strangest fates of any warship: struck by a U-boat torpedo in the Mediterranean, it was towed back to port and split into two halves which served as hulks (named HMS Pork and HMS Pine).