The 31-year-old Gomes reached a one-year deal with his hometown team on Thursday. According to a source close to the situation, the deal is for $1 million, with incentives that could bring it to $1.2 million.
To clear a spot on the 40-man roster, the A's designated infielder Adrian Cardenas for assignment.
With the addition of Gomes, a career .281 hitter against lefties, Oakland solidifies its outfield depth with a player who should nicely complement the left-handed-hitting duo of left fielder Seth Smith and right fielder Josh Reddick. He also offers manager Bob Melvin yet another option at the designated hitter spot.
Gomes, a Bay Area native, posted a .209 average with 14 home runs and 43 RBIs in a combined 120 games for the Reds and Nationals last year. The majority of his appearances came in left field.
He spent the first six seasons of his professional career with the Rays, and since 2003, he has compiled a lifetime .242 average and a .780 OPS. Gomes' most productive season came in 2010, when he hit .267 and drove in 86 runs while appearing in a career-high 148 games for Cincinnati.