| Ochil |
|
| Ochil and South Perthshire |
|
| Ogmore |
Full name (in Craig 3,4) Glamorganshire, Ogmore |
| Old Bexley and Sidcup |
|
| Oldbury and Halesowen |
|
| Oldham |
|
| Oldham Central and Royton |
|
| Oldham East |
|
| Oldham East and Saddleworth |
|
| Oldham West |
|
| Oldham West and Royton |
|
| Orkney and Shetland |
|
| Ormskirk |
Full name (in Craig 2,3,4) Lancashire, Ormskirk |
| Orpington |
Constituency of same name also existed 1918-1974ยน. |
| Orpington |
Full name (in Craig 3,4) Kent, Orpington. Constituency of same name reformed 1983. |
| Osgoldcross |
Full name (in Craig 2) Yorkshire (West Riding), Osgoldcross |
| Oswestry |
Full name (in Craig 2,3,4) Shropshire, Oswestry |
| Otley |
Full name (in Craig 2) Yorkshire (West Riding), Otley |
| Oxford |
|
| Oxford East |
|