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. He played in the 2000 Summer Olympics, helping to lead the U.S. to a fourth-place finish. O'Brien became a regular with the senior team under Bruce Arena, and came into his own at the 2002 FIFA World Cup. Playing in defensive midfield, he scored the opener in the 3–2 win over Portugal, assisted on Clint Mathis' goal against South Korea, and played every minute of every game for the United States.
