O.S.B.T
O. S. B. T means you’re such a lousy player that one bullet from a sniper makes you cry like a baby. It's usually for Fortnite players who get sniped and instantly quit.
"I got sniped by a kid and I’m O. S. B. T."
"That kid is O. S. B. T. He got me in one shot."
"I’m O. S. B. T. I quit the game after one bullet."