If someone is registered as an organ donor but their will says otherwise, which instruction controls in Ontario?

The registered donor consent is generally treated as the operative legal decision, not the will - and practically speaking, this makes sense given the timing involved, since organ and tissue donation decisions typically need to be made within hours of death, long before a will would normally be located and read. A conflicting instruction buried in a will is unlikely to even be discovered in time to affect what actually happens at the hospital.

This is described here as the general principle rather than a guaranteed rule for every conceivable circumstance - hospitals may still discuss the situation with family at the time, and practical, time-sensitive medical factors can influence what actually occurs. But as a starting point, registering as a donor through Ontario's official system is the more legally operative and more practically effective way to record this wish than relying on a will.

If your will and your donor registration genuinely conflict, for example if you've changed your mind since registering, the fix isn't to rely on the will; it's to update or withdraw your registration directly through Ontario's donor registration system, so the record that actually gets checked reflects your current wishes.