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| (o) | Open (boulder density) |
| O | Office |
| O | Old |
| O | Orchard |
| Obsy | Observatory |
| Oct | Octagonal |
| Oct | October |
| OE | Orignal Error |
| OG | Ornamental Ground |
| ON | Oven |
| OPP | Opposite |
| OR | Orchard |
| OR | Outcropping Rock |
| Orch | Orchard |
| Orkney | Orkney |
| OS | Out Stipple |
| OSBM | Ordnance Survey Bench Mark |
| OSGB | Ordnance Survey Great Britain |
| OST, OSTs | Ordinary Spring Tide(s) |
| OT, OTs | Ordinary Tide(s) |
| Oxon | Oxfordshire |
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