You've actually seen someone call someone else an oxymoron?
I have seen Bugs Bunny call someone (probably Daffy Duck) a "maroon" with my own eyes, but I'm skeptical that real people would seriously misuse "oxymoron" that way.
The court demands evidence for your claim that this is a common misusage.
Yep, the first time was on a "news" program, a few years ago. I don't remember which program, or which bimbo, but I was astounded. Her co-bimbo (male variety), didn't even blink. I expect he was just as stupid as her.
I've heard it a few times since, but haven't made a point of taking notes.