[nb 1] At the peak of his career, United States national teammate Landon Donovan called O'Brien the "best soccer player in the USA.

After being brought to attention of youth director Co Adriaanse, he had his first trial with Ajax at the age of 14 and signed an amateur contract with the Dutch giants in 1994 as a 17-year-old.

O'Brien played only one game with Chivas before suffering an injury with the U.S. national team during the 2006 World Cup, and was released by the club following the 2006 season.

[5] With the United States, O'Brien earned his first cap against Austria on April 22, 1998, but was an alternate for the U.S. squad for the 1998 FIFA World Cup.

After retiring from soccer, O’Brien returned to college and completed a doctorate in clinical psychology from Alliant International University.