Just did a little checking -- looks like 97 players have done it before. Only one name jumped out as a great player Will Clark in '86 -- a few others had good careers. A lot of them never did much, including a bunch for whom that was their only career HR. The only Oriole to do it before was Buster Narum in '63. Remember him? -- I sure don't. He was a pitcher and only lasted in the bigs a few years. I guess you could also include Bob Neiman for the Browns in '51 -- he was one of the better players for the poor Oriole teams of the 50s. A few other Orioles did it for other teams. Knuckleballer Hoyt Wilhelm for the Giants in '53 (remember Gus Triandos with the huge mitt catching him?) -- his only career homer. Also Benny Ayala for the Mets in '74 who was a valuable role-player for the great Oriole teams of the 70's.
But all that aside -- congrats to Montanez on a great, great debut. We all hope it continues!