Although released early in his career by his first club Middlesbrough, he was subsequently signed by Juventus in 1999. During three years with the Italian club, he played only occasionally for the first team and was loaned out to Lugano in Switzerland, lower division Italian clubs Crotone and Lecco, and Dundee United in Scotland. After leaving Juventus in 2002, O'Brien spent the rest of his career playing in Major League Soccer (MLS) for FC Dallas, Toronto FC and San Jose Earthquakes before retiring in 2008. He was named in the MLS Best XI in both 2004 and 2005, and was selected for the MLS All-Star Game for four years running.