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"Fake" means false.
"Sarcasm" means irony used with intent to scorn or convey contempt.
An "oxymoron" is a figure of speech in which two opposite ideas are joined to create an effect, such as "living death."
I can imagine sarcasm used playfully in flirtation, "Oh! You are so mean!" when the other is teasing - say, offering a strawberry and then holding it back. In that context, the words are ironic and could sound scornful, but don't. So the phrase could be described as fake sarcasm.
The opposite of fake is real.
And the antonyms of sarcasm are flattery, praise, compliment, and respectful or warmly sincere speech.
The ideas in "fake" and "sarcasm" are not quite equally opposite.
So I don't think the phrase "fake sarcasm" qualifies as an oxymoron.
In a way it almost does but, to me, it seems to just miss the mark.
Fake sarcasm? What does that even mean? Pretending to be sarcastic but not being sarcastic? What purpose would that even serve? Oh wait, I guess it is like pretending to be snotty but kidding around. Okay. Not an oxymoron.
hartfire, this is pretty good. I liked it up to a point. The oxymoron correction is valid. But sarcasm and scorn are not the same. And feigning scorn is not feigning sarcasm.
Now snotty and sarcastic are much more akin. So if I said, I DON'T FEEL LIKE IT OKAY????? and I was kidding, that is fake sarcasm.
Check the dictionary. Any dictionary.
There is no definition which leaves disrespect out of the intent in sarcasm.
Sarcasm is regarded by psychologists as a form of abuse and bullying.