| Band | Formed / started | Split up / ended | Note |
|---|---|---|---|
| O. | June 2021 | — | — |
| Oakenshield | 2007 | — | — |
| Oasis | 1991 | — | Noel & Liam Gallagher |
| Obey Robots | 2020 | — | — |
| Obsidian Key | 1 January 2010 | — | — |
| Occasion | 2025 | — | — |
| Ocean Colour Scene | 1990 | — | — |
| Ocean Red | 1 December 2010 | — | — |
| Oceans Ate Alaska | 2011 | — | — |
| Oceansize | 19 October 1998 | 25 February 2011 | — |
| October Drift | 1 January 2014 | December 2025 | — |
| Odium | 1998 | — | — |
| Odyssey | 1968 | — | originally US family act Lopez Sisters, now UK fronted by Steven Collazo |
| Of Solitude and Solemn | 2012 | 2018 | — |
| Oh Wonder | 2014 | — | — |
| Ohhms | 2014 | — | — |
| Old Corpse Road | 2007 | — | — |
| Old Forest | 1998 | — | — |
| Old Radio | April 2011 | — | — |
| Old Swing | 2017 | — | — |
| Olive | 1994 | 2001 | UK dance/breakbeat/trip‐hop group |
| Omega | 1983 | — | — |
| Omega Tribe | 1981 | — | — |
| Ommadon | 2008 | — | — |
| Omnimalevolent | 2 September 2025 | — | — |
| On Parole | 1979 | — | — |
| One Dimensional Creatures | 2023 | — | — |
| One Thousand Violins | 1985 | 1989 | — |
| One Way System | 1979 | — | — |
| Onslaught | 1983 | — | — |
| Ooberman | 1997 | 2010 | — |
| Opia | 2022 | — | — |
| Oppress. | 2016 | — | — |
| Opus Kink | 2017 | — | — |
| Orange Goblin | 1995 | 17 December 2025 | English heavy metal band |
| Orange Juice | 1976 | 1984 | — |
| Orchestral Manoeuvres in the Dark | 1977 | — | — |
| Original Raving Fatties | 1979 | 1979 | — |
| Original Sin | 1982 | 1983 | — |
| Orpheum | 1 January 2010 | — | — |
| Osibisa | 1969 | — | — |
| Ostara | 1999 | — | — |
| Oswald | 15 May 2005 | — | — |
| Otalgia | 1985 | — | — |
| Out of Darkness | 1968 | 1974 | — |
| Out of Sight | 2001 | 24 January 2011 | — |
| Outfit | 14 January 2011 | — | — |
| Outl4w | May 2004 | — | — |
| Outside Your House | 1 April 2014 | — | — |