1) Abbreviation for "On God" meaning to heavily agree to something, and show that agreement by mentioning the life of God.
2) Misspelling of "OMG" which is an abbreviation for Oh My God/Gosh/etc.
1)
P1: "My favorite part about the party was how hyper the DJ was"
P2: "ONG DUDE"
2)
P1: "ONG I JUST GOT TOLD VERY BAD NEWS ABOUT MY PARENTS"
P2: "You misspelled OMG, dumbass."
P1: "Why does that matter...?"