Recently my son (who is 4) started say "Oh My God!" for everything. Dropped shoes, pencils, he found a peanut, little sister was throwing books, EVERYTHING. I am not an over the top religious person, but I don't say it that often and to be honest, it was starting to bother me. So I took him aside and tried to explain to him that even though it isn't a bad thing to say, he doesn't need to say it for everything.
Well, my mil is staying with us for mothers day and she is not used to being around kids 24/7. Especially kids who will repeat everything you say. Every time she says OMG, he stops and says "Grandma, did you say Oh My God?" in a scolding tone. I have to admit that it is freakin hysterical, especially now that she has been here one night and is getting exsaperated with it. I've tried telling him it's ok for once in a while, but he LOVES scolding Grandma.