At the start of the Second World War Onslow was on the Plans Division of the Naval Staff, with a combat interlude in 1940 on an unsuccessful attempt to evacuate the Belgian government and gold reserves from Bordeaux during the Fall of France, nearly becoming prisoner of the Germans. He next became captain of the destroyer in 1941 in the role of defending Russian convoys, as well as the convoys to Malta. His services on the former convoys earned him the initial award of his Distinguished Service Order (DSO) and the Soviet Order of the Red Banner. He took over the anti-submarine training establishment HMS "Osprey" in 1943 and went on to be captain of the 4th Destroyer Flotilla in November, in which capacity he earned the third of his three bars to his DSO in the attack on a Japanese base at Sabang, Sumatra.