Can I be sued for defamation for stating an opinion rather than a fact in Ontario?

Generally, no, if the statement is a genuine expression of opinion rather than an assertion of fact. Defamation claims target false statements presented as fact, and a clearly recognizable opinion, even a harsh or unflattering one, is generally protected, particularly where it is grounded in facts that are true or otherwise properly disclosed to the audience. The practical difficulty is that the line between opinion and fact is not always obvious from the wording alone.

Courts look at how a reasonable reader or listener would understand the statement in its full context, not simply whether the speaker used words like I think or in my opinion when making it. A statement phrased as an opinion can still be treated as an implied factual assertion if it suggests the speaker knows undisclosed facts that would justify the opinion, which is a common way that seemingly protected commentary can still expose someone to a defamation claim. This is why fair comment, discussed elsewhere, is the recognized defence for genuine opinion, rather than opinion being automatically immune from a claim in the first place.