As far as Oh My God...I don't say that...not even OMG. I do like Oh my godless, and might start saying that. Usually I just say Holy shit, or fuck or crap. Or Jesus fucking christ. I know...I have a ridiculously terrible potty mouth, and I love it!
I try to avoid it. My family used to say Gesundheit when I was growing up, so that's my default reaction. But I've had some people not know what on Earth I was saying, I guess it's a regional / family thing.
If I'm speaking to someone who knows another language, I'll usually say it in that language if I know it (Salud — for "health" and not "salvation"! — in Spanish, "À tes/vos souhaits" in French), or see if I can get away with not saying anything. If I can't avoid it, I'll say Gesundheit or rarely Bless you, but never "God bless you".