Steam locomotives operated in the United Kingdom with names beginning with O.
| Name | Railroad, # | Builder, s/n | Build date | Whyte type | Notes |
|---|---|---|---|---|---|
| Oakley Grange | GWR 6823 | 4-6-0 | |||
| Oakley Hall | GWR 5936 | 4-6-0 | |||
| Oberon | LMS 25277 | 4-4-0 | |||
| Ocean | LMS 5730 | 4-6-0 | |||
| Octavia | GWR | 0-6-0 | |||
| Ogmore Castle (1st) | GWR 5080 | GWR Swindon | 5/1939 | 4-6-0 | Renamed Defiant in January 1941. |
| Ogmore Castle (2nd) | GWR 5056 | 6/1936 | 4-6-0 | Renamed Earl of Powis in September 1937; withdrawn November 1964. | |
| Ogmore Castle (3rd) | GWR 7007 | 7/1946 | 4-6-0 | Renamed Great Western in January 1948; withdrawn February 1963 | |
| Ogmore Castle (4th) | GWR 7035 | 8/1950 | 4-6-0 | Withdrawn August 1965. | |
| Oliver Cromwell | BR 70013 | Crewe | 6/1951 | 4-6-2 | Wd 8/1968; preserved at Great Central Railway. More information: Great Central Railway. |
| Olton Hall | GWR 5972 | 4-6-0 | |||
| Olympus | GWR | 0-6-0 | |||
| Ontario | LMS 5554 | 4-6-0 | Became BR 45554 | ||
| Orient Line | SR 21C8 | 4-6-2 | |||
| Orion | GWR | 2-2-2 | |||
| Orion | LMS 5691 | 4-6-0 | |||
| Orion | SDR | 4-4-0ST | |||
| Ormonde | LNER 2556 | 4-6-2 | |||
| Orpheus | GWR | 0-6-0 | |||
| Orson | GWR | 0-6-0 | |||
| Osborne | SR 19 | 0-4-4T | Operated on the Isle of Wight | ||
| Oscar | GWR | 2-4-0 | |||
| Osiris | GWR | 0-6-0 | |||
| Osiris (1st) | SDR | 4-4-0ST | |||
| Osiris (2nd) | SDR | 4-4-0ST | |||
| Ostrich | GWR | 2-2-2 | |||
| Ostrich | GWR | 2-4-0 | |||
| Ostrich | GWR | 2-4-0T | |||
| Ostrich | SDR | 4-4-0ST | |||
| Otho | GWR | 2-4-0 | |||
| Ottawa | GWR 3399 | 4-4-0 | |||
| Overton Grange | GWR 6879 | 4-6-0 | |||
| Ovid | GWR | 4-4-0ST | |||
| Owen Glendower | BR 70010 | Crewe E479/220 | 5/1951 | 4-6-2 | Wd 9/1967; scr 1/1968. |
| Owl | SDR | 0-4-0WT | |||
| Oxford | GWR | 0-6-0 | |||
| Oxfordshire | LNER 253 | 4-4-0 |