Stands for "Manual Orifice Sex". Describes when a man manually cuts a hole in someone and sticks his cock into the hole. This is the fastest way to get HIV.
Guy 1: "Hey dude, did you here about the one guy who killed that girl by M.O.S?"
Guy 2: "No dude I didn't, that is sick."
Guy 1: "Yah well at least he got HIV."