| O | Birmingham |
| OA | Birmingham |
| OB | Birmingham |
| OC | Birmingham |
| OD | Exeter |
| OE | Birmingham |
| OF | Birmingham |
| OG | Birmingham |
| OH | Birmingham |
| OJ | Birmingham |
| OK | Birmingham |
| OL | Birmingham |
| OM | Birmingham |
| ON | Birmingham |
| OO | Chelmsford |
| OP | Birmingham |
| OR | Portsmouth |
| OS | Glasgow (Wigtownshire until 1974; Stranraer until 1981) |
| OT | Portsmouth |
| OU | Bristol (Hampshire until 1974) |
| OV | Birmingham |
| OW | Southampton |
| OX | Birmingham |
| OY | Croydon (changed to London (NW) from 1967) |