He served in the Second World War as a lieutenant colonel on the staff of the Tactical School in 1940 before becoming a general staff officer at Southern Command in 1940.[4] He was appointed commanding officer of 70th Battalion, Welch Regiment in 1941 and then became a general staff officer with responsibility for operations at Eastern Command in 1942.[4] He was deputy director of Military Operations at the War Office later in 1942 and then brigadier on the General Staff at 9th Army HQ in the Middle East from 1943.[4] He was made director of Military Operations for Middle East Land Forces in 1944 and then returned to his post as brigadier on the General Staff at 9th Army HQ in the Middle East in 1944.[4] He became chief of staff to General Sir Bernard Paget at General Headquarters, Middle East Command, in 1945.[4] On 3 August 1945 Oliver was promoted to the acting rank of major-general.[9]