On April 30, 2001, Leighton signed with the Seattle Sounders, where he had a successful season by posting 11 goals and seven assists, All-League honors. Following the completion of the USL season, he signed with the Sacramento Knights of the World Indoor Soccer League for the remainder of their season. In 2002, the Sounders loaned him to the Clash. In 2002, he had another successful campaign with the Sounders where his 13 goals and 11 assists helped the Sounders claim the regular season championship. His efforts also earned him All-League recognition, Assist Leader honor, and was named the USL A-League MVP. He then signed on loan from the Sounders with the Milwaukee Wave of Major Indoor Soccer League for the 2002–2003 season. He played two winter indoor seasons, 2002–2003 and 2003–2004 with the Wave. The Sounders signed O'Brien to a three-year contract before the 2003 season, but he played only three games in Seattle before moving on loan to Bodens BK in the Swedish Second Division. He played 23 games, scoring six goals and adding nine assists. In 2004, he returned to Boden for the 2004 season, but lost most of the season with a groin injury. On November 14, 2005, the Wave traded O'Brien's rights to the California Cougars in exchange for Pat White, but did not play for the Cougers.