If my will says I want to donate my organs, can my next of kin override that wish in Ontario?

Generally, no - under Ontario's organ and tissue donation framework, a valid documented consent, most importantly registration through Ontario's organ donor registration system, is treated as the operative legal consent, and next of kin do not have a general legal veto once that consent is properly registered. That said, this is the general principle rather than a guaranteed outcome in every situation - hospitals will often still discuss the decision with family at the time, and practical, time-sensitive medical circumstances can affect what actually happens.

A will is not the most reliable place to record this wish in the first place, because a will is often not located, read, or even known about until after a funeral has already happened - organ and tissue donation decisions typically need to be made within hours of death. Registering your wish directly through Ontario's official donor registration system, and telling your family and your named Power of Attorney for Personal Care about it, is far more likely to actually be acted on in time than a wish buried in a will.

If honouring this wish matters to you, treat the will as a backup, not the primary record - register your decision and talk to the people who will actually be present at the time.