it's a shortened word that stands for "Oh My God" and it starting to become a phrase actually used in REAL-life human interaction when having a conversation. It's sad seeing grown ass people using that phrase when talking to others....
Me: can i borrow your shoes?
Sister: no, i need them
*sneaks in and takes the shoes from closet*
Sister: What?!! take my shoes off!, Oh-emm-gee!"