Castro was recalled from Double-A Bowie on Wednesday.
Castro made his first appearance of the season for Bowie last week, picking up the win after hurling four innings of one-run ball. This, along with his big-league experience during his time with the Rockies, prompted the Orioles to bring him to the majors following their 13-inning marathon with the Tigers on Tuesday night. It's unclear how long Castro will stick with the big-league club, but his live arm could play up if he can show some control over his offerings. To make room for him on the active roster, Donnie Hart was optioned to Triple-A Norfolk.