From April to August 1942, Tunku Osman trained with the
Gloucester Regiment as a recruit and later joined the 80th
Reconnaissance Regiment. He was then sent to undergo cadet training at the Highland Fieldcraff Centre in
Scotland. He underwent further training at
Bovington and was seconded to the
Royal Armoured Car Regiment. In 1944, he entered the OCTU at
Barmouth,
Wales in December 1944 to undergo training as a
cadet officer. In 1945, Tunku Osman joined the
Force 136 and was commissioned as
second lieutenant (May 1945) and was absorbed into the General List. He continued
parachute training and investigations at
Calcutta, India. He spearheaded the Malay Guerrilla Group when Force 136 was abolished after surrender of Japan. 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. In July 1958, Tunku Osman was promoted to
lieutenant colonel and took command of the 2nd Battalion Malay Regiment. He was the first
Malay commanding Officer of the battalion. In May 1960, Tunku Osman was promoted to the rank of colonel while he was attending the Joint Services Staff College and in June 1960 was promoted to brigadier general and took command of 2nd Malayan Infantry Brigade. In July 1961, Tunku Osman was made "Brigadier of The Army" at the Ministry of Defense.
Appointment as Chief-of-Staff of the Armed Forces When General Tan Sri
Sir Rodney Moore retired as the
Chief of Staff of the Armed Forces, Tunku Osman was promoted to
major general and appointed the Chief of Staff of the Armed Forces. He was the first Malayan to hold the highest rank in the Malaysian Armed Forces. Tunku Osman retired from his position on 24 November 1969. ==Honours==