I am an Orioles fan and don’t (well, almost never) call the Orioles the O’s. First, the apostrophe doesn’t make sense. If anything, it should be the Os (plural instead of possessive). Hardly any other teams do this. The only one I can think of is the A’s. We don’t call the Phillies the P’s. We don’t call the Yankees the Y’s.
That’s enough of an old man rant for today.