Ontario home purchase: is a signed Agreement of Purchase and Sale binding?

Yes. In Ontario, once an Agreement of Purchase and Sale is in place, it is a binding legal contract. Conditions in the agreement can still matter greatly, including financing, inspection, title and closing terms. Buyers and sellers should understand those conditions before signing or waiving them. Official sources: RECO and Ontario law. Guide: https://getinstantlaw.com/property-buying-selling/contract-problem