O'Hagan was born in Belfast in 1934 and became active in republican revolutionary politics in the city from an early age. He was imprisoned in Crumlin Road jail in his native Belfast from 1956 to 1960 at the time of the Irish Republican Army Border Campaign.
On his release from imprisonment O'Hagan was enrolled in the London School of Economics from where he graduated in 1964. He subsequently returned to Belfast and was appointed to the post of Senior Lecturer in Sociology in Stranmillis College of Teacher Education, now known as Stranmillis University College.
In 1967 O'Hagan was one of the founding members of the Northern Ireland Civil Rights Association (NICRA) and played an important role in organising many of the campaign's major demonstrations and in the development of NICRA into a serious political machine which forced the British government of the time to make major political concessions.
Des O'Hagan was one of a large number of republicans who were arrested and interned without trial in August 1971. He quickly began documenting the ill-treatment of prisoners by the British authorities and had these articles smuggled out of the Long Kesh internment camp. They were soon serialised by the Irish Times newspaper as Letters from Long Kesh. His articles played a significant role in exposing the torture and ill-treatment of prisoners at the camp and drawing international attention to them. The "Letters from Long Kesh" were finally published in book form, by Citizen Press, in September 2012 to mark the 40th anniversary of their serialisation. 
On his release from internment O'Hagan became once again involved in the official republican movement and played an important role of its development into the Workers' Party. He was for a time editor of The Irish People and served on the party's Central Executive Committee for many years during which he was Chairman of the party's Education Committee. He also contested a number of elections on behalf of the Workers' Party in the South Down constituency.
Des O'Hagan died on 5 May 2015, aged 81.
- The Lost Revolution: The Story of the Official IRA and the Workers' Party, Brian Hanley and Scott Millar, p. 222. ISBN 1-84488-120-2
- Letters from Long Kesh, Des O'Hagan, Citizen Press, Dublin, ISBN 978-0-9572835-0-3