Atheist spirituality is not an oxymoron. It makes perfect sense.
The word atheist itself means "without god". It does not mean "without religion or superstition". When you label yourself an atheist, you are simply stating that you don't believe in god, not that you also lack belief in any other supernatural beliefs.
By definition, buddhists are also atheists. They do not believe in a god, however they are highly spiritual. Buddhism is an atheistic religion. Those that are part of the Christian faith partake in a theistic religion, meaning they do believe in a god.
Hence whenever someone asks me what I "am" (religion-wise), I never say atheist. I say I'm devoid of any religion, and I do not adhere to any group. I don't want to be grouped in with people that think likewise, I just want to be myself.