Bailey had been out since March 30 with a strained right forearm and was cleared to return after making his fourth rehab appearance in the minors Friday.
"He's had enough rehab assignments and he feels like he's ready," Oakland manager Bob Geren said. "It makes a good bullpen even better. Off the bat, I'll use him in some non-save situations and go from there. When he's ready to assume the closer job then I'll give it back to him."
Bailey, the 2009 AL Rookie of the Year, has 51 career saves over two seasons and his return adds another right-handed arm to Oakland's bullpen. It also gives Geren a right-left option for closers once Bailey is settled in.
Left-hander Brian Fuentes has been the A's closer while Bailey has been out and earned his 10th save in 12 tries during Saturday's 4-2 win over Baltimore. But Fuentes was openly critical of Geren last week when he said there was a lack of communication between the manager and members of the bullpen.
Getting Bailey back will likely move Fuentes into a setup role, though he could still be used as a closer against left-handed hitters if Geren wants.
"It just adds a lot of depth," Geren said of Bailey's return. "When he's in midseason shape he's one of the best in the business."
To make room for Bailey the A's optioned left-handed reliever Bobby Cramer to Triple-A Sacramento. Cramer was called up from the minors on Friday when Oakland traded David Purcey to Detroit but did not pitch in either of the first two games against Baltimore.