In 1946, Tunku Osman joined the British Military Administration (BMA) in Malaya and served in the British Army in Malaya. In 1947 Tunku Osman joined the Royal Malay Regiment (1 February 1947) and was commissioned as lieutenant and assigned to the 2nd Battalion Malay Regiment. He was promoted to captain in February 1951 and later to the rank of major in 1955. The same year, Tunku Osman was sent to attend a Senior Officers course at Devizes, Wiltshire. In 1958, Tunku Osman was assigned to the 6th Battalion and the 7th Malay Regiment as Assistant Commanding Officer.