Perhaps you are right.... and I must say I don't ask anyone if they are an Atheist or a Christian, or in fact any other Religion or Faith. I can usually tell in forums who is who by the responses to my postings; for some reason, Atheists don't like what I post and accordingly, their replies reflect their feelings. This is regardless of what I write about....<quoted text>
I've never gone out of my way to ask people who say, "Oh God," if they are an atheist or a Christian because it is a habit instilled by society that has nothing to do with the existence of God. I do not say it because the people around me do not say it often. People who are around people who say it often are, however, more likely to say it.
We have a show down here "The Biggest Loser" and I think we followed on from the American version, but they face their fears towards the end of the show, and a lot of them have fear of heights. They had them swinging high in the mountains and all of them were calling out "Oh God" or "Oh my God" and there had not been any references to God during the whole show. They were not all in the same place at the same time, either, but perhaps you are right, they had heard it and it simply came to mind when no other words would describe their experiences :)
I believe also that tears are a way of expressing our feelings when mere words will not suffice; which is why we can cry for so many different reasons. I also love the emotions of our emotions..