The joke I've heard that I think this one is based on is this:

A priest, a pastor, and a rabbit walk into a bar. The bartender says, "What'll ya have?" and the rabbit says, "I dunno. I'm probably a typo."

Although I've seen the rabbit reply "I dunno. I'm only here because of autocorrect" which I like better.

But I think OP's version here is even better. It makes "type O" actually topical and make sense in the joke's framework/world.