Oakham Castle
Odell Castle
Odiham Castle
Ogle Castle
Ogmore Castle
Okehampton Castle
Old Beaupre Castle
Old Buckenham Castle
Old Castleton
Old Hall Camp
Old Sarum Castle
Old Warden Quince Hill
Old Wardour Castle
Olivers Battery
Ongar Castle
Orcop Castle
Orford Castle
Ormside Hall
Oswestry Castle
Oversley Castle
Oxburgh Hall
Oxford Castle
Oxwich Castle
Oystermouth Castle
