The Athletics announced they are trading left-handed infielder Kevin Merrell, a former first-round pick, to the Royals for Bailey. A former first-round pick himself, Bailey is 7-6 with a 4.80 ERA in 18 starts this season.
Bailey has hit his stride in recent weeks. He is 3-1 with a 3.35 ERA and 37 strikeouts in 43 innings across his last eight starts.
Bailey is 74-83 with a 4.58 ERA since debuting with the Cincinnati Reds in 2007. He emerged as one of the Reds' top young pitchers, throwing two no-hitters and going 42-34 with a 3.77 ERA in 110 starts from 2011 to 2014 before injuries took their toll.
Bailey was scratched from his start with the Royals on Sunday. The Athletics have not announced when Bailey will make his team debut.
Oakland entered play Sunday with a 52-41 record. The Athletics are six games behind the first-place Houston Astros in the AL West Division.
Oakland is half a game ahead of the Cleveland Indians for the American League's second wild card spot. The Royals are a dismal 32-61.