What is the word for when you call something the opposite of what it is eg calling a BIG man 'tiny'?

oxymoron

The above answer is incorrect!

Correct AnswerIt's "dysphemism". refer to the usage of an intentionally harsh Word or expression instead of a polite one; they are rough opposites of euphemism.

An oxymoron expample would be "I have a cute uglydog."