Mr O'Hagan was abducted by the Provisional IRA's infamous South Armagh brigade in 1989 after they had murdered the two most senior members of the RUC killed in the Troubles.
His name was on a contact list recovered by the IRA from the bodies of Chief Supt Harry Breen and Supt Bob Buchanan, killed as they returned from Dundalk Garda Station. Mr O'Hagan, a chatty and amiable journalist with an eye for a strong story, had struck up a relationship with the chief superintendent, who worked out of Armagh RUC Station.
Some months after the two RUC officers were killed, the IRA contacted Mr O'Hagan for what seemed a routine face-to-face meeting. He was bundled into a car, blindfolded and taken to a house in the Border area between south Armagh and Dundalk.
One of his captors told him he had been involved in the torture and murder of IRA informants and made it clear the same fate might await Mr O'Hagan.
However Mr O'Hagan managed to convince his captors that he had not worked as an RUC informer and was released unhurt with a warning. Mr O'Hagan's worst enemies were the dissident loyalist group, the Loyalist Volunteer Force (LVF).
In 1992 he exposed the leader of the LVF as the sectarian assassin Billy Wright, who lived only a few miles from Mr O'Hagan's home in Lurgan.
After Wright vowed to kill him, Mr O'Hagan was advised by the RUC to leave Northern Ireland. He did so, working for his employers, the Sunday World, in Cork and Dublin for about a year before deciding that the threat against his life had subsided and that he was safe to return home to his wife, Marie, and their three daughters.
Mr O'Hagan was often close to controversy and was the source of information used in the Channel 4 Dispatches documentary on the supposed "Committee" inside the RUC alleged to have been involved in loyalist sectarian assassinations.
The documentary led to a series of court cases in which evidence was given that Mr O'Hagan had briefed the principal witness in the case in a shed in the back garden of his house in Clara Street in Lurgan.
The Channel 4 programme alleged that a secret committee of professional people, security forces and loyalist paramilitaries colluded to murder Catholics and republicans.
Mr O'Hagan said in evidence in one of the court cases arising from the Dispatches programme that his conviction in the early 1970s for possessing firearms resulted from his agreeing to collect a number of rifles for the Official IRA.
He had served 3-1/2 years in the Maze Prison, regarding himself as a political prisoner, and took an Open University course which he later continued at the University of Ulster.
He started working for the Sunday World in 1987. He had earlier worked for the political magazine Fortnight, and shortly before joining the Sunday World, worked in a freelance capacity in the Belfast office of The Irish Times.