He began his senior career under Billy Young at Bohemians where he was also a youth international representing his country at the 1983 UEFA European Under-18 Championship where Ireland were knocked out despite not losing a game.
[5] With the club not recovering sufficiently after losing their first three league games of the season, a League Cup loss to Southampton saw Alex Ferguson take over from Atkinson as manager in November 1986, with O'Brien making his debut a couple of games into Ferguson's tenure, in a 2-0 win over Leicester City at Old Trafford on 20 November 1986.
However, he played a major role as Newcastle won the First Division title in 1993, making 33 appearances and scoring six goals.
O'Brien found himself surplus to requirements as Newcastle rejoined the top flight and subsequently made a £350,000 move to Tranmere Rovers after a successful loan period.
O'Brien played for the Republic of Ireland national under-19 football team that qualified for the 1983 UEFA European Under-18 Championship where despite remaining unbeaten in a group that included eventual winners France they were eliminated.