| Site name | Country | County | Nearest town | Site type | Access | Classification (girth in metres) | View on map |
|---|---|---|---|---|---|---|---|
| Old Sodbury Hill Fort | England | Gloucestershire | Bristol | earthworks | Yes | Ancient 4m-5m | View On Map |
| Old Wardour Castle | England | Wiltshire | Shaftesbury | historic site | Yes | Notable | View On Map |
| Old Winchester Hill | England | Hampshire | Fareham | woodland | Yes | Notable | View On Map |
| Old Windsor | England | Berkshire | Windsor | churchyard | Yes | Lost | View On Map |
| Oldbridge House – Drogheda | Ireland | Co. Meath | Drogheda | historic site | Yes | Ancient 4m-5m | View On Map |
| Oldland | England | Gloucestershire | Bristol | churchyard | Yes | Ancient 5m-7m, Lost | View On Map |
| Oldmeldrum | Scotland | Aberdeenshire | Inverurie | garden | No | Notable | View On Map |
| Orchard Portman | England | Somerset | Taunton | churchyard | Yes | Notable | View On Map |
| Orchardleigh | England | Somerset | Frome | churchyard | Yes | Ancient 4m-5m | View On Map |
| Ormiston | Scotland | East Lothian | Edinburgh | historic site | No | Ancient 5m-7m, Notable | View On Map |
| Orpington | England | Kent | London | churchyard | Yes | Notable | View On Map |
| Osmaston | England | Derbyshire | Ashbourne | churchyard | Yes | Ancient 5m-7m | View On Map |
| Osmington | England | Dorset | Weymouth | churchyard | Yes | Ancient 4m-5m | View On Map |
| Ospringe | England | Kent | Faversham | churchyard | Yes | Notable | View On Map |
| Oswestry | England | Shropshire | Oswestry | churchyard | Yes | Ancient 5m-7m | View On Map |
| Oswestry – The Hayes | England | Shropshire | Oswestry | garden | No | Lost, Notable | View On Map |
| Oswestry – Welsh Walls | England | Shropshire | Oswestry | churchyard | Yes | Notable | View On Map |
| Otford | England | Kent | Sevenoaks | churchyard | Yes | Ancient 4m-5m | View On Map |
| Otham | England | Kent | Maidstone | churchyard | Yes | Lost | View On Map |
| Othery | England | Somerset | Langport | churchyard | Yes | Notable | View On Map |