Unless you're extremely wealthy, like a certain tech tycoon who needed a liver, so registered in every OPO in the nation, flew to the transplant hospital that had his liver and bought a house to recuperate in for a few months, thus shortcutting the system and cheating someone else out of a liver they really needed. And you can get away with it, too, as UNOS and the OPO's are not allowed to know who is registered in any OPO unless they have a specific agreement in place. Less wealthy people can't afford to register everywhere or fly at a moment's notice to a distant hospital.
UNOS has an entire department dedicated to identifying transplant recipients because they can register under an alias, as Dick Cheney did for his heart (that actually makes sense, as no one could be accused of weighting the scale one way or the other).