The O and P classes and the first emergency flotillas

The initial plan simplified the J-derived hull and reduced the armament to speed building. The P class used high-angle quick-firing 4-inch guns, as did some O-class ships, while other Os carried 4.7-inch guns. The five-gun 4-inch example includes a quadruple pom-pom, a single quadruple 21-inch torpedo bank, four depth-charge throwers and roughly 70 depth charges. Some hulls were laid down in late December 1939 and others followed in 1940. O-class ships later took part in the Battle of the Barents Sea.