I thought of a fallacy

This isn't a fallacy. Expressions (like "oh my god") are not arguments, statements of fact, or anything other than ways of conveying a feeling.

I literally put it in an argument form:

If A says B

And B is contrary to C

Then Not A should say C

(Based on the idea of what conforms more)

I don't think there was any implication whatsoever that. "oh my god" is an argument, the idea was a critique of how the phrase conforms to who based on the context on who uses it as an exclamation.

The form of the argument would be similar to something like, "It makes much more sense for people who feel pain to scream than people who don't feel anything and arbitrary scream".

I mean if you see a person get punched in a stomach really damn good and hard and they scream it kind of makes sense, wouldn't you find it odd if a person who is just randomly standing screams in pain and then just stops screaming without expressing it physically either?

Just to throw another point into the discussion, many believers in God will say that when an atheist says "oh my God" it's because deep down inside they actually do believe despite their insistence they do not. It's like the "there are no atheists in fox-holes" argument. 

However, I was in a bunker, on the DMZ at Khe Sanh.  I was/am an atheist, and I remain one.  If my example proves the exception, than it follows there are more “atheists in fox holes “.  Anti-theists, even…