O'Brien also represented the Republic of Ireland under-16 team and was part of the squad that won the 1998 Under-16 European Championship.

O'Brien's footballing career began very promisingly, when at the age of 18 he signed a contract with Middlesbrough of the FA Premier League.

In search of consistent playing time, O'Brien, made the unconventional decision to move to America to join the Dallas Burn.

Due to a knee injury during a training session, which was later aggravated during a friendly match, O'Brien was limited to 13 games in the 2007 season.

In return, Toronto FC received San Jose's first round pick in the 2009 draft plus an undisclosed amount of money.

Following O'Brien's strong performance and evident leadership in 2008, the San Jose Earthquakes stunned their fans by failing to pick up his contract option for the 2009 season.

[citation needed] O'Brien achieved success with the Republic of Ireland national youth sides, including winning the 1998 Under-16 European Championship alongside John O'Shea.