Christenson attended Pepperdine University and was drafted by the Oakland Athletics in the 10th round of the 1995 Major League Baseball draft. He played in his first professional season that year for the Class A Short Season Southern Oregon A's, batting . 190/. 286/. 247. In 1996, he played for Southern Oregon and the Class A West Michigan Whitecaps. In 1997, he advanced to the Class A-Advanced Visalia Oaks, Double-A Huntsville Stars, and Triple-A Edmonton Trappers. He began the 1998 season at Triple-A, but was called up to the Athletics and made his major league debut on April 20. Christenson started the 1999 season in Oakland, but was sent to the Triple-A Vancouver Canadians for a month. He returned to the big league club in late May, and batted . 209/. 305/. 306 for the season. He spent the entire 2000 season with the A's. He played seven games at the start of the 2001 season with Oakland, going hitless in four at bats before being sent to the Triple-A Sacramento River Cats, for whom he batted . 171/. 216/. 271.