He soon became a regular in the Bradford City first team, and helped the Bantams to promotion to the FA Premier League during the 1998–99 season.

He was a regular in the Bradford City side during its two years in the Premier League, forming a partnership with club captain David Wetherall in central defence.

One of his first games saw him score an equaliser in a north-east derby with Sunderland, while in his first full season the club secured a place in the Champions League qualifiers.

He fell down the Portsmouth first-team list with the arrival of Sol Campbell and the good form of club captain Linvoy Primus.

[7] O'Brien quickly settled at the Reebok Stadium and impressed the fans in his first few appearances as part of the Wanderers squad including a dominating performance in the UEFA Cup tie against FK Rabotnicki and in the Premier League against Tottenham Hotspur.

O'Brien after captaining the team in a Uefa Cup game against Sporting CP he said that he is having the time of his life at Bolton.

[8] In May 2008, his efforts were recognised by Wanderers supporters when he was named as The Bolton News Player of the Season following a poll held by the newspaper.

[19] After a two-game spell on the sidelines with injury,[20] O'Brien returned to Leeds' starting lineup against Leicester City.

[26] O'Brien picked up an injury in the FA Cup replay against Arsenal and subsequently missed the game against his former club Portsmouth.

[32] He was given a torrid time against his former club Bradford City and was substituted to loud jeers in the second half with Leeds 2–1 down.

[39][40][41] After manager Simon Grayson was sacked, O'Brien played his first game for Leeds as a substitute against Coventry City.

[44] On 1 August 2012, O'Brien moved to Canadian MLS club Vancouver Whitecaps FC on a free transfer from Leeds United.

[46] After the conclusion of the 2014 campaign, O'Brien entered into contract talks with the Whitecaps, but the two sides were unable to reach an agreement.

[49] O'Brien's performances for Newcastle led to him being called up to the Republic of Ireland national football team, earning his first cap against Estonia in Tallinn in 2001.