[1] Born in Tacoma, Washington, O'Brien grew up in the Pacific Northwest, graduating from Federal Way High School in 1994.

[4] 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.

His efforts also earned him All-League recognition, Assist Leader honor, and was named the USL A-League MVP.

[8][9] 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.

[12] Salt Lake loaned O'Brien back to Seattle several times during the season before releasing him on August 19, 2005.

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.

Having been unable to secure a professional contract, O'Brien signed with Tacoma Tide of the USL Premier Development League for the 2009 season.

[15] As of January 2019, O'Brien is the technical director for Pacific Northwest Soccer Club in the Seattle, Washington area.