Good question, which requires, I think, that any atheist clarify the reason he doesn't believe in a god. There aren't too many reasons. Either you have not made up your mind yet, in which case I would suppose you're really agnostic, or you do not believe in the existence of a god because you are convinced that there is no god in which to believe. I suppose we could argue around the clock about the finer points of the word "belief" and whether this constitutes a belief that god does not exist or something else. Perhaps this is why dictionary definitions usually use a stronger term than simple absence of belief. Most atheists would, I think, be found in the camp of those who assert that there is no god.
I realize this is a loaded subject, because owing to the cultural bias in our language it is hard to find a way to define the term that does not assume as a default that there is a god to deny, and that disbelief in god is a failure of belief or a position contrary to the norm, but there's a social sense in which that is the case. Of course god is in a special category, being something that cannot be demonstrated or proven in the way anything else is. To say you do not believe in god is not quite the same thing as saying you do not believe, say, in bigfoot. I do not believe in bigfoot because I see no reason to, but obviously if a bigfoot knocked on my door I would change my mind. The existence of a bigfoot would not turn my view of reality upside-down and force me to reevaluate everything I believe. My not believing in bigfoot is stronger than simply being an agnostic and saying "I don't know," but obviously weaker than that of someone who bases large portions of his world view on the impossibility of there being one. This is because the possibility of bigfoot is not supernatural, but also because it is not very important. I do not believe in Jehovah because I am convinced that such a god is impossible and irrational: I assert that this is so, and to change that position would be to change my life. That of course doesn't make me an atheist in the total sense, if I am reserving the agnostic option for impersonal, vague, wishy-washy second-string deistic compromises, but that's another story.
Perhaps it's just a difficult thing to be, or to define, a "pure" atheist in a society and a language which is permeated with theism. It will be a long time before a term like "godless" is neutral.
Those Oxford folks should get together with themselves. The Oxford Universal Dictionary adds to the standard definition (the one you don't like) one who "denies God morally," which is not the same thing, and the DK Illustrated Oxford (pretty worthless one) doesn't even have the word!