| Site name | Country | County | Nearest town | Site type | Access | Classification (girth in metres) | View on map |
|---|---|---|---|---|---|---|---|
| Oakhills Coppice SSSI – Long Crichel | England | Dorset | Blandford Forum | woodland | No | Ancient 5m-7m | View On Map |
| Oakley Copse | England | Wiltshire | Wilton | woodland | No | Ancient 4m-5m | View On Map |
| Oceno | Spain | Asturias | Cangas de Onis | churchyard | Yes | Notable | View On Map |
| Ockham | England | Surrey | Guildford | garden | No | Ancient 4m-5m | View On Map |
| Ockley | England | Surrey | Horsham | woodland | Yes | Ancient 4m-5m | View On Map |
| Octeville l’ Avenel | France | Manche - Normandie | Quettehou | churchyard | Yes | Ancient 5m-7m | View On Map |
| Odcombe | England | Somerset | Yeovil | churchyard | Yes | Ancient 5m-7m | View On Map |
| Odstock | England | Wiltshire | Salisbury | footpath/road | Yes | Notable | View On Map |
| Oerley Hall – nr Rhydycroesau | England | Shropshire | Oswestry | garden | No | Ancient 4m-5m | View On Map |
| Offas Dyke – nr Maes-Treylow | Wales | Powys | Presteigne | historic site | Yes | Ancient 7m+ | View On Map |
| Offham | England | Kent | Maidstone | churchyard | Yes | Ancient 5m-7m | View On Map |
| Offranville | France | Seine-Maritime - Normandie | Dieppe | churchyard | Yes | Ancient 7m+ | View On Map |
| Offwell | England | Devon | Honiton | churchyard | Yes | Ancient 4m-5m | View On Map |
| Okeford Fitzpaine | England | Dorset | Sturminster Newton | churchyard | Yes | Ancient 5m-7m | View On Map |
| Old Alresford | England | Hampshire | Winchester | churchyard | Yes | Ancient 5m-7m | View On Map |
| Old Church Hotel | England | Cumbria | Penrith | former religious site | Yes | Ancient 7m+ | View On Map |
| Old Colwall | England | Herefordshire | Great Malvern | woodland | No | Ancient 7m+ | View On Map |
| Old Colwyn | Wales | Conwy | Colwyn Bay | footpath/road | Yes | Lost | View On Map |
| Old Coulsdon – Devilsden Wood | England | Greater London | Caterham | woodland | Yes | Notable | View On Map |
| Old Coulsdon churchyard | England | Greater London | London | churchyard | Yes | Notable | View On Map |