Born in Stepney, in the East End of London, O'Brien started work in the printing industry when he was fourteen years old.
[1] O'Brien served in the Merchant Navy and was on board the Rangatiki in the Jervis bay incident then the Royal Air Force during World War II.
After the war, he returned to printing, becoming active in the Labour Party and his union, the National Society of Operative Printers' Assistants (NATSOPA).
[2] He was elected as the union's assistant general secretary in 1964, and that year also became a governor of the London College of Printing, later chairing the organisation.
In 1976, NATSOPA passed a motion to this effect,[4] and O'Brien forged a close working relationship with Bill Keys of the Society of Graphical and Allied Trades (SOGAT).