Can the owner of a vacant or unoccupied property be sued if someone is injured there in Ontario?

Yes. An owner of a vacant or unoccupied property does not escape occupier responsibilities simply because no one is actively using the space; the Occupiers' Liability Act still applies to whoever retains control over the property, and an owner of vacant land or an empty building can be liable if a hazard they knew or should have known about - an unsecured structure, a hidden excavation, deteriorating stairs - causes an injury to someone lawfully, or even unlawfully, present on the property.

The duty owed can be lower toward someone trespassing on vacant property than toward an invited visitor to an occupied one, following the general reduced trespasser standard discussed elsewhere, but it is not eliminated entirely, particularly where the owner knows or should know the property tends to attract entry despite being unoccupied - curious passers-by, children, or others drawn by an obviously interesting or accessible vacant site. Owners of vacant property are generally expected to take reasonable steps, such as securing entry points and addressing known dangerous conditions, rather than assuming that a lack of ongoing activity on the site removes their responsibility altogether.