Tin Oo joined the army on 26 February 1946 as a Second Lieutenant in Burma Rifles Battalion. He reached the ranks of Lieutenant on 7 January 1947, Captain on 27 September 1948 and served as executive officer at Armed Forces Training Headquarters. On 22 June 1949, he was transferred to No.1 Burma Rifles Battalion as Company Commander. He was promoted to the rank of Major on 25 January 1950 and became Deputy Battalion Commander (2IC) of No.1 Burma Rifles Battalion and took over the position of acting Battalion Commander on 27 November 1951.
Tin Oo was promoted to Lieutenant Colonel on 21 January 1954 and became Commander of 4th Infantry Brigade on 30 May 1957. He was then transferred to Army Officer Training School as Commandant on 13 September 1957. Throughout 1958 and 1961, Lieutenant Colonel Tin Oo served as Battalion Commander for No. 14 Infantry Battalion (from 18 November 1958), No. 2 Burma Rifle Battalion (from 16 February 1961) and after his promotion as Colonel, he became acting Commander for No. 13 Infantry Brigade (from 20 February 1961).
He was then given the command of South West Regional Military Command and promoted to the rank of Colonel on 14 February 1963. On 19 September 1964 he became Commander of Central Regional Military Command. He was then promoted to the rank of Brigadier General and became Deputy Chief of Staff (Army) on 20 April 1972. On 8 March 1974 he was promoted to the rank of General and became Commander in Chief of Tatmadaw. He was armed forces Commander in Chief during the bloody crackdown on student protests surrounding the funeral of former UN Secretary General U Thant.
During his military career, General Tin Oo was awarded with Thuya medal, prestigious award for gallantry and bravery in the face of the enemy that can be awarded to members of Myanmar Armed Forces. He led both tactical and strategic campaigns against the Karen National Union as well as the Communist Party of Burma and various ethnic armed groups, especially in the north and east of the country.
On 6 March 1976, As per Order no. 26/76 issued by the Council of State, Tin Oo was forced to retired from his position as Commander in Chief of the Armed Forces of Burma. According to the official explanation released by the then ruling party, Burma Socialist Programme Party, General Tin Oo was forced to retire because Dr. Daw Tin Moe Wai, his wife, broke the rules and regulations laid down for the spouses of commanding officers of the Tatmadaw by accepting numerous bribes, thus affecting General Tin Oo's position as Commander in Chief of the Armed Forces.
After his forced retirement, he was accused of high treasons against the armed forces (Tatmadaw), the party (BSPP) and the state. He was subsequently arrested and tried for the alleged withholding of information concerning a failed coup-d'état against General Ne Win and the Council of State. On 11 January 1977, Judge U Ohn Maung, Chairman of Divisional Justice Committee for Yangon Division sentenced him to 7 years hard labour and imprisonment according to Crime Against State and High Treasons Act 124. Tin Oo's subsequent appeal for this judgement on 20 August 1977 was summarily dismissed by Judge Soe Hlaing, of Council of People Justice, the equivalent of the Supreme Court, and upheld the judgement handed out by Yangon Division Justice Committee. Colonel Hla Pe, commander of Northern Regional Command, Colonel Maung Maung, Colonel General Staff and Colonel Myo Aung, commander of Southwest Regional Command were also dismissed and the former two were imprisoned along with General Tin Oo.
He was released under general amnesty in 1980, after which he studied and received a degree in Law. On 2 September 1988, he became the Vice President of the National League for Democracy (NLD) and on 20 December, the President of NLD. From 20 July 1989 he was put under house arrest and from 22 December 1989, he was imprisoned for three years.
On 30 May 2003, Tin Oo, travelling with the caravan of Aung San Suu Kyi, leader of the NLD, was attacked in the northern village of Depayin by a government-sponsored mob, murdering and wounding many of his supporters. Tin Oo was taken into detention along with Aung San Suu Kyi and was initially held in prison in Kalay in northwestern Myanmar. In February 2004 he was brought back to his home in Yangon, where he is actually held under house arrest. The junta extended his detention by one year in February 2007, 2008, and 2009. The last of these extensions was in violation of the rule of Burmese law, but no explanation was given by the junta. He was released from House Arrest on February 13, 2010.