McCoy, 26, appeared in 14 games with the Tigers as a rookie this season and allowed six runs and 21 hits in 14 innings for a 3.86 ERA. He walked 13 batters and struck out 11.
His walk-to-strikeout ratio was much better in the minors, where he went 1-0 with a 3.95 ERA in nine games at Double-A Erie and 2-0 with a 2.59 ERA in 21 games at Triple-A Toledo. McCoy walked nine and struck out 34 over 45 innings.
Left-handed hitters were 5-for-24 (.208) against McCoy in the majors, and right-handers were 16-for-37 (.432).
McCoy is 19-20 with a 4.49 ERA in eight minor league seasons, with 23 starts among his 254 appearances.
The Nationals selected McCoy in the 10th round of the 2007 First-Year Player Draft out of Sahuaro High School in Tucson, Ariz. The Tigers signed him as a free agent in November.
“He’s a lefty reliever our scouts like and who’s effective versus left-handed hitters and has a good arm,” said Orioles executive vice president Dan Duquette.
The Orioles now have 33 players on the 40-man roster.
McCoy gives the Orioles bullpen flexibility because he’s got a minor league option remaining.