FIRE services from across the county attended the former Doris Watts Care Home on Milestone Road, Carterton, yesterday after a passer-by spotted smoke.
Crews from Bampton, Witney, Abingdon, Faringdon and Oxford were called to the scene.
Police Sgt Philippa Payne said: “As it was disused, we are treating this as suspicious and the cause is under investigation.”
Fire and rescue service station manager Mike Adcock said: “There was no-one inside. There was severe damage to two first-floor rooms and smoke damage to the rest of building.”
Harry Watts, 63, owns the building. He said: “I had turned off all of the electrics, so I am certain that it was arson.
“Buildings don’t catch fire by themselves. The fire crews and police were splendid.”
The building is up for sale. Mr Watts says it has potential to be converted into a hostel or B&B.
The fire broke out on the first floor of the east wing.
Police closed off Milestone Road to the public at 8am, and opened it again at 12.30pm.
The care home closed after an inspection from the Care Quality Commission last March revealed a failure to provide adequate care.