September 1942 saw the running of the next Arctic Convoy, PQ 18. The events of PQ 17 resulted in the decision to give the convoy a very strong escort. As well as the normal close escort, for most of the convoy's route it would be accompanied by the Escort carrier , and by a "Fighting Destroyer Escort" consisting of 16 destroyers led by the light cruiser . This force was backed up by a cruiser covering force, with more distant cover provided by the battleships of the Home Fleet. "Onslow" formed part of the Fighting Destroyer Escort, which joined the convoy on 9 September. The convoy came under heavy attack by German submarine and aircraft from 13 September, and on 14 September a Swordfish aircraft from "Avenger" spotted the German submarine on the surface. "Onslow" was despatched against the submarine and carried out a series of depth charge attacks over three hours, sinking the submarine. The carrier and Fighting Destroyer Escort remained with PQ 18 until 16 September, when it transferred to the westbound Convoy QP 14 to escort it through the area of most danger, with "Onslow" leaving QP 14 on 25 September. In total, 13 ships out of 40 from PQ 18 were sunk, with the escort sinking three U-boats and claiming 41 German aircraft shot down. Three more merchant ships and a fleet oiler were lost from QP 14.