Oh, my God! (OMG) is used to express surprise, excitement, disbelief, etc. It is informal or is more of slang. OMG! That bag is fabulous. or OMG! You look amazing. Other ways of saying OMG are: Holy Moly! (My favourie way of saying OMG) Goodness Gracious Me! Oh Gosh!

ummmm I think most of time we just directly say OMG and Wow huhhuh,that's quite common and universal.or you can try: 我的天哪。that's quite common also.