Moore, who was outrighted by the Astros earlier this month, played in 72 games last season for Houston, starting games at five different positions -- third base, first base, right field, second base and left field.
"I had quite a few offers and Oakland called me and talked to me and basically there, guys in my situation have been known to get pretty good opportunities to play," he said. "Also, I'm going into my 12th season [in pro ball] and I've never really played anywhere remotely close to home (Southern California) and that was pretty exciting, too."
Moore, 29, was a first-round Draft pick by the Tigers in 2002. He's a career .242 hitter with 16 homers and 53 RBIs in 152 Major League games with the Cubs, Orioles and Astros.