The A’s of Oakland, of course.
But if someone says the O’s or the M’s won,I know they are talking about the Baltimore/Seattle MLB teams.
Who else?
The Jay’s is a stretch since it is usually spelled out.
The Js of course.
(Toronto Blue Jays)
I refer to the Yankees using one finger. Does that count? 
Hehe,edited before I saw your post
And you edited your comment with the Jays after I posted my response…
touché
Yes, I see that,but I did not see your post.
Do the papers in Toronto use J’s or Jays?
Always Jays.
I think A, O, and M may be it.
The NY Giants are sometimes referred to as the G-men, if that counts.
The Boston Bruins are referred to as the Bs.
The U (University of Miami Hurricanes)
I’ve always thought this was a ridiculous nickname. They even have a U on the football helmets. I’m a little biased, since I can’t stand Miami’s football team. 
The University of Minnesota teams (the Golden Gophers) have a big M logo, but the institution itself is known (at least in Da Cities) as Da Yoo.
Yes. Also allowed is spitting after saying the name.
The Orioles are occasionally called the Os. There aren’t many that would be nationally recognized like the As and the Jays.
+1
And occasionally the Celtics are called the C’s, but mainly by bored TV announcers, not fans.
How about 1.5 letters?
University of Washington teams: U-dub
I think so too. Like who the hell wouldn’t be “The U”?
Likewise, the San Francisco Giants are occasionally called The G-Men. Not often, though.