According to Webster's an oxymoron is a combination of contradictory or incongruous words. Here's an opportunity to add some of your favorite ones. I'll start it off.
Why are a "wise man" and a "wise guy" opposites?
Why do "overlook" and "oversee" mean opposite things?
If love is blind, why is lingerie so popular?
Why does "fat chance" and "slim chance" mean the same thing.
If work is so terrific, why do they have to pay you to do it?