| Code | Name | Region |
|---|---|---|
| GBOTH | Otham | KEN |
| GBOTL | Otley | LDS |
| GBOTY | Otley | SFK |
| GBOBO | Otterbourne | HAM |
| GBOTR | Otternish, North Uist | ELS |
| GBOTT | Ottershaw | SRY |
| GBOSM | Ottery Saint Mary | |
| GBOGB | Oughtibridge | SHF |
| GBOBR | Oulton Broad | SFK |
| GBOND | Oundle | NTH |
| GBOUK | Outer Skerries | ZET |
| GBOVF | Ovenden | DNC |
| GBOVR | Over | CAM |
| GBXOV | Overseal | DBY |
| GBOVE | Overton | HAM |
| GBOWE | Ower | HAM |
| GBOLN | Olney | BKM |
| GBOMA | Omagh | OMH |
| GBOGR | Ongar | ESS |
| GBONH | Ongar Hill | ESS |
| GBOS9 | Openshaw | MAN |
| GBORB | Orby | LIN |
| GBORP | Orchard Portman | SOM |
| GBORH | Orford | SFK |
| GBORK | Orkney | ORK |
| GBORM | Ormesby | MDB |
| GBOSA | Ormesby Saint Margaret | NFK |
| GBOMK | Ormskirk | LAN |
| GBOPG | Orpington | BRY |
| GBORT | Orsett | ESS |
| GBORS | Orston | NTT |
| GBORO | Orton | CMA |
| GBOGP | Osgathorpe | LEC |
| GBOMT | Osmaston | DER |
| GBOML | Osmotherley | NYK |
| GBOSS | Ossett | WKF |
| GBODM | Odiham | HAM |
| GBOTN | Offerton | SND |
| GBOCY | Offord Cluny | CAM |
| GBOBM | Ogbourne Maizey | WIL |
| GBOKH | Okehampton | |
| GBOLD | Old | NTH |
| GBODL | Old Dalby | LEC |
| GBODO | Old Dornie | HLD |
| GBOHN | Old Hutton | CMA |
| GBOKK | Old Kilpatrick | WDU |
| GBOMN | Old Malton | NYK |
| GBOOY | Old Romney | ESX |
| GBOTF | Old Trafford | MAN |
| GBOWG | Old Whittington | DBY |