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.