There is no professional baseball team with the name "O's" (and why is it possessive, anyway?) either in their articles of incorporation or their contract with Major League Baseball. If you pursue the conclusion of shortening the name of the Baltimore baseball team, then when they play the Tigers, are they playing the T's? And when they play the Twins, are they playing the T's again? How about the Angels and the Athletics? Are they playing the A's both times? Rangers and Red Sox? R's??

What about when they play the Blue Jays?

Just call them Baltimore. That is the city they play in. That is the city they represent. Though they have, for some reason, been embarrassed to wear that name on their uniform for decades.

And, Analise, I have lived in Northern VA for 12 years. I went to my first major league game (a doubleheader vs Pittsburgh) at Ebbets Field. I have gone to the ballpark in Baltimore to see the "Y's" bash them at least a dozen times.