It is not necessary to introduce a new idea or concept with "Oh, my God" or, if you feel the need, please use the teenybopper varient, "ohmigod!" which indicates to the listener (me) that what you're saying isn't actually that important or pivotal.
"Oh my God" has become the new "fuck". Remember when every sentence started with "fuck" no matter how important or trivial the subject. Ranging from the "Fuck, I got pocket lint on my jeans." to "Fuck, I can't believe that guy just dumped 18 tons of fertilizer on my convertable," no sentence or thought was complete without its very own "fuck."
And now we have the more or less offensive, depending on your perspective, "Oh my God." "Oh my god, the cafeteria was out of a blue cheese!" "Oh my god, look at the size of the dinosaur that is about to eat me."
Oh my god, people, grow up. Just say what you need to say. Don't preface it with stuff. And don't bother G-d with your petty concerns. This public service announcement brought to you from the people who actually have prayers that need to be answered. 8-)