Abbreviation for "Oh my God." Most commonly used online and in chat rooms. However, people who can not be bothered or are too stupid to string three words together and must use the letters instead use it in everyday life as well.
"omg last night was so f-ing cool!"
"like oh em gee did you hear that jane broke up with bob and bob is going out with jane's brother's uncle's aunt's cousin's dog's hamster's mom? like omgzs!"