I don't see how Americans don't consider "oh my god" taking God's name in vain? I mean, if you are not talking to God, why insist on saying oh my God? Why not say "wow" or "oh" or "ick" depending on the situation.
Imagine if someone said "oh my Willie" or "Oh my ladybug" .... it wouldn't make much sense would it?
Oris: You could tell her that it makes you feel bad, and ask her kindly not to do it around you, then leave it up to her if she uses it elsewhere