The exclamation "oh my God" is so common, even among those who do not believe in God, that to most people, it seems like a normal and innocent phrase. Even many Christians, sadly, will think nothing of it. Whenever I hear people say it, I often want to ask them, which God?
If you're a Christian saying "oh my God", you're actually acting contrary to your own religion. Scripture condemns the vain use of the Lord's name. If you're going to use His name, it better be in a correct context. For me, there is even a dilemma about whether it is ok to repeat the term being discussed today, and I decided yes, it's ok to repeat it, but only because I am rebuking it. But answer me this: If you respect someone, would you use their name as a cuss word? I don't shout my mother's name when I stub my toe. Do you? And do you imagine God enjoys it when you do it to Him? That, my brethren, is a sin that is nailed to the cross. Jesus died so that God can forgive the sin of abusing His name. If it's that serious, why would we continue using it improperly?
But maybe you're an atheist. Maybe, when you exclaim "oh my God", you aren't referring to any in particular. But I have bad news: When you say "oh my God", you're right. He's your God whether you like it or not. And He will judge every idle word.
No matter your religious affiliations, the phrase "oh my God" is not innocent. It is useful for only one thing: Exposing us as the sinners we are. See, the very existence of this unique term proves the uniqueness of God, even in our own hearts and minds. We don't use fictional characters, each other, nor even our rulers, as cuss terms. No one says "oh my boss" or "oh my king". The thought just never occurs to us. But it seems the devil forces us to tell the truth through our lying lips: Oh my God.
Your God hung on a cross for such insolence. Every unholy thought, every idle word, every evil deed, incurs the wrath of God upon ourselves. But that wrath was taken on our behalf on the cross. By confessing Jesus as Lord, and believing in our hearts God raised Him from the dead, we will be saved from the wrath to come, and granted an everlasting inheritance in His Kingdom. If you must say "oh my God", direct it towards Him, and ideally place a great big "thank you" in front of it.