O.P.P.
Early 90's hip hop slang, popularized by Naught By Nature. In the song, O.P.P. stands for "other people's property" but colloquially it's known as "other people's pussy." This is inferred by the song lyrics and the general pondering of one being willing to cheat on their significant other.
"You down with O.P.P.? Yeah you know me!"