In covering with this volume the 0-Z war-built vessels we hope to bring attention to a much neglected destroyer group. It has been popular comment that these classes compare badly with their foreign contemporaries but it is not simply size together with torpedo and gun power that constitutes a successful design. Seaworthiness, and above all, reliability, play a vital part, and these were characteristics justly laid claim to by the British destroyers. The Os and Ps in particular are often thought of as the low-water mark of British destroyer design and construction but of all the war-built classes they gave more service during the period of hostilities than any other in spite of their low-angle main armament, which was of doubtful value against aircraft; the paucity of close-range AA guns; and their limited anti-submarine capabilities.