Castro was recalled from Double-A Bowie on Friday.
After spending less than a week in the minors, Castro will rejoin the big club to offer bullpen depth in the absence on Stefan Crichton (shoulder), who was placed on the disabled list in a corresponding move. Castro owns a 4.09 ERA and 1.46 WHIP in 11 innings out of the Orioles' bullpen this season. He has combined for 24.1 innings in six appearances at Double-A, so he'll offer some length out of the bullpen for the Orioles.