Several times throughout the movie "Guardians of the Galaxy Vol. 2" step-siblings Gamora (Zoe Saldana) and Nebula (Karen Gillan) exclaim "Oh my God!" (capitalization from BluRay close captioning), usually when something explodes or collapses around them.
I don't know the Guardians of the Galaxy mythos outside the two movies. I understand if Peter Quill said that (maybe he even did, can't recall), since he does come from Earth and could have been raised if not in then at least around some Christian religion, but which God do they believe in in the Andromeda galaxy?
Or is it just the movie writer's oversight?