Oh my god, you’re a Valkyrie! I used to want to be a Valkyrie when I was younger.
There are script issues here. Why would Thor say, “Oh my God”? I’m pretty sure Thor doesn’t deal with a single God in any religious sense and he and his friends do admit to spending their wasted youth popping onto less evolved planets and BEING their gods for kicks and giggles. In NO social context does this make sense. Neither do contractions unless someone is speaking with a race that uses them.
I’m not bothered by contractions – every language has shortened forms. The “oh my God” is a serious problem, though. (We all have to be creative about that in fic, using Norns and Ymir and Yggdrasil for the standard Christian religious oaths…)