The only rigid rule for capitalizing “God” in dialogue and thoughts is that you do so when using it as a pronoun: “Joe, God won’t like that.” Beyond that, you can let your character decide. Some characters say “Oh my god!” as a generic expression with no thought to religion at all. For them, lowercase works. They aren’t directly invoking God. For a character with strong religious beliefs, the word “God” almost certainly has religious association whenever it’s used, thus you’d capitalize it in her dialogue and thoughts—if she’d utter the name in vain at all. For her, “God damn it” would really be a plea to God on some level. For characters in the middle, “oh my god” is likely generic so lowercase works unless you just plain don’t like it, but you can avoid your discomfort or offense to readers by using the casual “oh gawd” or picking a different exclamation or curse. There are plenty! Or, how about just skipping the curse and using body language and prop interaction instead? A guy silently punching a wall can be richer than any curse word.