OB Share definition Flag Onion Booty. Used to describe a phat-ass bitch with a phat ass. Reference www.onionbooty.com Yo Greg, 10 o'clock your time. Check out OB on that one son ... Daaamn! OB by James Werdsbond December 9, 2008 0 0 Get merch Get the OB mug. Buy Now — $32.95 See more merch