The statement your mother made makes absolutely no sense.
Even if Atheism WAS a religion - it would still make absolutely no sense as you can still be part of a religion and NOT believe in a Deity.
She failed on both counts. But I'm sure you still love her :)
I suspect that she didn't didn't think it through. An oxymoron requires two mutually contradictory aspects of a whole concept. Atheism is a single aspect, the absence of a belief in a deity. There's nothing for it to be self-contradictory with. To do so, you'd have to add an additional aspect, such as an atheist who believes in a god.
Perhaps she's simply falling for the common misconception that atheists actually believe in a god, but are just angry at him/her.
I suspect the "contradiction" in her mind is between a. god exists and b. you think he doesn't. That would actually be an example of irony, but I don't think she understands that.
Religious people think that the existence of god is "obvious" and "undeniable" because there is "no other explanation" for the universe. They treat the two issues as one in the same and engage in very circular reasoning regarding them.
I have heard this argument it goes something like this.
A-Theist=no-god. To say there is no god would mean you know everything making you god.
That's about it. Bad logic. We should really expect this though.
I would ask her to define "oxymoron" and go from there. Like Dave G said, it necessarily entails two contradictory concepts. This is why oxymorons consist of two words, like "silent scream" or the humorous "military intelligence."
Now, she could be trying to argue that atheism is somehow inherently paradoxical by using the fallacious argument that rejecting God presupposes God. The argument generally states: "In order to reject God, you assume that there is a God that can be rejected." This argument relies upon mistaking the rejection of a belief in God with the rejection of God himself. Rejecting a belief does not presuppose the subject of the belief.
However, I would let her explain her reasoning to you before mounting a defense. It is better to let someone spell out their own fallacious argument. Sometimes, the act of actually explaining a concept is enough for the speaker to realize the flaws themselves.
This is why oxymorons consist of two words, like "silent scream" or the humorous "military intelligence."
Or "Microsoft Works"