A supporter of Enoch Powell, he attempted to organise a 'Powell for Premier' movement following the Rivers of Blood speech. O'Brien gained a reputation for working towards unity on the far right, establishing contacts not only with the NDP, but also the Monday Club, the Union Movement, the Integralists led by white Russian George Knuppfer and a number of local anti-immigration groups, with the NF ultimately absorbing a number of such groups. Following internal wranglings within the party, O'Brien was appointed leader of the NF in 1970, following the resignation and removal of A. K. Chesterton (who had brought O'Brien in to be NF Office Manager).
Initially seen as a compromise candidate (after the rebellion against Chesterton, no one was willing to take the post), he soon set about trying to modernise the party and clashed with John Tyndall and Martin Webster over the issue, who had backed the O'Brien candidacy because they thought erroneously that he could be easily manipulated. The simmering conflict came to a head when O'Brien accused Webster of working with the Northern League, which had been proscribed in the NF. O'Brien moved to expel Webster but failed to get Tyndall's backing leading to open conflict. During the resulting struggle O'Brien briefly departed from the scene to go on honeymoon and during his absence the pro-Tyndall contingent made moves to expel a number of his supporters.
Although the NIP initially looked like it might challenge the NF, Tyndall's party was galvanised by the arrival in Britain of Uganda's Asian population, who had been expelled by Idi Amin. O'Brien did not return to the political arena after this although he contributed to the British nationalist journal Candour. He died suddenly on 21 September 1982.
Within one month of the broadcast, Tyndall was fired as NF Chairman. O'Brien should not be confused with the John O'Brien involved with the White Nationalist Party, as the latter is still alive.