McGwire spent the past three seasons in the same job with the St. Louis Cardinals, where he starred during parts of his 16-year major league career. During McGwire's tenure, the Cardinals led the National League in batting average (.269) and on-base percentage (.337), ranked second in runs (2,263) and fourth in slugging percentage (.416).
He worked with All-Star sluggers Carlos Beltran, Lance Berkman, Matt Holliday and Albert Pujols, while helping develop 2011World Series MVP David Freese.
The job is a homecoming for McGwire, who is from the Los Angeles suburb of Pomona and currently lives in Orange County. He played college baseball at Southern California and was a member of Oakland's 1989 World Series champion team.
McGwire replaces Dave Hansen, who was let go last month and hired by the Seattle Mariners for the same job.