They served as convoy escorts in World War II, and some were subsequently converted to fast second-rate anti-submarine frigates in the 1950s.

They were in low-angle mounts which could elevate to only 40 degrees, and were additionally fitted with a 4-inch anti-aircraft gun in place of one set of torpedo tubes.

The second group had 4-inch (102 mm) guns in high-angle mounts and were fitted to act as minelayers; they could be recognized by the flat "beaver tail" stern over which the mines were dropped.

The latter proved to be outdated, and were replaced by 20 mm Oerlikon guns as they became available, with a total of six single mounts eventually being carried.

The P class were repeats of the O class, armed entirely with 4 inch guns, in high-angle mounts fitted with a new tall design of shield which did not require the ships to lose a set of torpedo tubes to take on further AA guns.