Can an insurer void my policy for a misrepresentation I made when I applied in Ontario?

Yes, potentially, but not for just any inaccuracy. Under Ontario's Insurance Act and general insurance principles, an insurer may be able to void a policy, or deny a specific claim, if you made a material misrepresentation on your application - meaning an inaccurate statement about something significant enough that it would have affected the insurer's decision to issue the policy at all, or on what terms. A trivial or immaterial inaccuracy generally isn't enough on its own to void coverage.

The insurer generally has to show both that the statement was actually false and that it was material - that a reasonably careful insurer, knowing the truth, would have declined to issue the policy or would have charged more or added conditions. Innocent misstatements are treated differently than deliberate misrepresentation in some respects, and the specific circumstances of how the application was completed, including what you were actually asked and what you knew at the time, matter a great deal. If your policy is being voided or a claim denied on this basis, obtaining your original application and any notes about how it was completed is an important first step in assessing whether the insurer's position actually holds up.