However, he was later expelled from the IRA after joining the Northern Irish civil service, a job which required swearing an oath of loyalty to the British monarchy.

[1] He was imprisoned in Crumlin Road jail in his native Belfast from 1956 to 1960 for attempting to rescue a Republican from police custody while being treated at Mater Infirmorum Hospital.

[1][3] On his release from imprisonment, O'Hagan was enrolled in the London School of Economics, where he was influenced by Marxist theorist Ralph Miliband, graduating in 1964.

[1][2] Between his time in London and the growing influence of left-wing members of the IRA/Sinn Féin such as Cathal Goulding, Tomás Mac Giolla and Seán Garland, O'Hagan became a committed Communist.

[4] On his release from internment, O'Hagan resigned from his post in the overwhelmingly Protestant Stranmillis and once again became involved in the official republican movement and played an important role of its development into the Workers' Party.