QUESTION: When is an O a Q?
ANSWER: When it's a prolapsed vowel . . .
Okay, to understand this riddle, you obviously need to know what a prolapse is. Get a dictionary. I'll wait. Then you'll need to be able to visualize figuratively as well as literally, but mostly figuratively.
Having done all that, you will understand this riddle.
Of course, by then, you'll no longer think it's funny, and you'll resent all the work you went through to understand it.
Moral: Some humor is hard.