Petit, 33, put together his best season in 2017, posting a 2.76 ERA in 60 appearances (one start) for the Angels. He struck out 101 batters and walked 18 in 91 1/3 innings.
Prior to joining the Angels on a one-year deal, Petit pitched for the Marlins, D-backs, Giants and Nationals, helping the Giants win the 2014 World Series with a 1.42 ERA in 12 2/3 innings that postseason.
The A's entered the offseason targeting relief pitching to help a young starting rotation that posted a 4.74 ERA over 877 2/3 innings. With the starters often unable to pitch deep in games, Oakland's bullpen was taxed, posting a 4.