On March 29, 2005, he signed with the expansion Real Salt Lake of Major League Soccer. At the time, he was still under contract with the Sounders. The Sounders released him to Salt Lake in exchange for the team to play an exhibition against Seattle. Salt Lake loaned O'Brien back to Seattle several times during the season before releasing him on August 19, 2005. O'Brien immediately signed with the Sounders. His free kick in the 2005 USL-1 Final started what would become Seattle Sounders tying goal. His kick was redirected twice before Maykel Galindo scored Seattle's goal in a 1-1 draw with Richmond Kickers. O'Brien then scored one of four penalty kicks for Seattle as they won the 2005 Championship in penalty kicks. Leighton signed back on with Seattle for the 2006 season, and in 2007, he helped the Sounders claim the double. He remained with the club for its final season as the team folded at the end of 2008 due to Seattle Sounders FC joining Major League Soccer.