He began to write professionally at the age of 16. In 1938 he joined the British Army, and during the war served as an NCO in mobile radio detachment (3 Corps) of Royal Corps of Signals in the 8th Army. He saw active service in Persia in 1942, after which his unit was moved to Syria, Egypt, the Western Desert, and Italy, and he was with forces that went into Greece in October 1944. After the war, O'Donnell returned to civilian life and began to script comic strips, including an adaptation for the "Daily Express" of the James Bond novel, "Dr. No". From 1953 to 1966 he wrote for "Garth", and from 1956 to 1962 "Romeo Brown" (with Jim Holdaway as an artist).