The word "god" can be used as both a proper and common noun.
There is a difference between saying "Oh my god!" and "Oh my God!". When you use a capital(G), then that's a proper noun. When you don't use a capital(g), then that's a common noun.
I wasn't expecting to give y'all an english class during the weekend, but alas, here we are.
So in short...the english language is really confusing, thanks for coming to my TED Talk.
aint readin allat