Hunter is 1-1 with 11 saves and a 6.06 ERA in 19 appearances. He took over for the traded Jim Johnson as Baltimore's closer at the beginning of the season but lost the job after blowing a save opportunity against Detroit on May 13.
Hunter pitched two times after that, the last time on Tuesday night in Pittsburgh. The move, announced Thursday, is retroactive to Wednesday.
The Orioles filled Hunter's spot in the bullpen by recalling right-hander Preston Guilmet from Triple-A Norfolk.
Guilmet has appeared in two games with Baltimore this season, striking out three in 1 2-3 scoreless innings.