No, not MANDATORY organ donation. Just having the necessary information on an database so that if someone dies in an accident or violently, any people waiting for an organ might get one if there's a match.
MOST people would die normally and would never have their organs touched. Those people that die in excellent health would have organs TAKEN from them to provide for people who need them.
If organ donation was non-voluntary, there would be no harvesting of organs (like kidneys) from people in third world countries. There'd be enough to go around to provide for those that need them without having to do terrible things like that.
I fully understand that this is a difficult concept for people to accept. You're born with all your organs, why shouldn't you go to the grave with all your organs. And, the answer, quite simply, is that someone else needs them more than you will. And, that's a good answer so far as I'm concerned.