O's 8-25 vs. Marlins since that team's inception. Thought that was a bit surprising, but then I thought about how the Marlins have won two World Series in that time-span and O's have won none.
Personal note: The first foul ball I ever caught at The Yard was vs. Marlins on 7-16-2000. Was a foul tip. I had just sat down in my seat in the first inning and told my friends, "I got you covered, I have my glove." The a foul tip from a Marlin batter went right into my glove. I was in the Club Section just to the first base side of the press box, first row. Same batter hit another just out of my reach.