Adj. An abbreviation of the classifieds/sales term "Or best offer". Used in the context of describing a man or woman who will take whatever he/she can get in the way of romantic companionship.
Melissa? She's a drunk with a lot of mileage on her. She's truly OBO.
Adam has slept with all of his friends' ex-girlfriends and now he's been hanging around that creepy, snaggletoothed, bettie page impersonator's house. Isn't she almost 50? Officially OBO.