During the Second World War, Oldfield joined the British Army. Initially a sergeant in Army Field Security (which was absorbed into the Intelligence Corps in 1940), he was commissioned as a second lieutenant in the Intelligence Corps in July 1943. Most of his wartime service was in Egypt at the headquarters of SIME (Security Intelligence Middle East) in Cairo. This was primarily a counter-intelligence organisation, the role of which was to detect hostile agents in the region and counter their activities.
