The sound of someone eating a large item of food in one go...
om!...that dohnut was goood
OM is short for OMG... when you have no time for G. OM!
Less syllables when you don't have time.
It sounds cuter.
It can also be an acronym for "Oh My!"
"OM! She is cray cray!"
"Hey girl! OM! I love that skirt, where did you get it?"