Good one, SojoMojo! :-)
My own experiences with teens and young people over the past couple of decades have exposed me to the many ways "oh my god" can be used in a conversation. There's the emphatic form Razzak cites, there's the quick quip, "ohmygod," meaning something like "what now?" or "not now!" or "are you serious?" "In this sense, "ohmygod" can be used equally well in response to "that guy you like is on the phone," or "that guy you can't stand is on the phone." There's the down emphasis at the end meaning something like, "I can't BELIEVE how frustrating that person/this situation is," then the up emphasis and I'm not sure what that means because I tune out when I hear upspeak. "Oh my God" has become as ubiquitous as "yes," "no," "please," in the sense that it would be possible to stage an entire conversation using only that phrase between two people, using just enunciation and emphasis to convey the nuances of meaning.
On a student exchange trip to Britain I chaperoned in 1996, one girl's reaction to everything was "oh my GOD!" From the way they drive on the left to seeing a Jaguar on the motorway to the places we stayed, it became the tagline for the entire trip. You can hear it on the videos. I knew it was common in my circles, but I hadn't realized until I read this thread how common it's become worldwide. Interesting.
I say "oh my God" even though I know it's blasphemous. Going for a euphemism doesn't change what you're actually saying, so I figure I might as well say what I mean and not sugar it up with "oh my gosh." But I don't use most Anglo-Saxon epithets.