| Oaken Palace Records | United Kingdom | August 1, 2012 | — | — |
| OBR | United States | January 1, 1988 | — | 1 |
| Ocean Butterflies Music | Singapore | January 1, 1986 | — | 2 |
| Ocora | France | — | Original Production | — |
| Octubre | Spain | January 1, 2008 | — | 1 |
| Ode Records | United States | 1967 | Original Production | — |
| Odeon | Germany | 1904 | Original Production | — |
| Odeon | Germany | January 1, 1903 | — | 25 |
| Odin Records | Norway | January 1, 1981 | — | 1 |
| Ogdoo Music Group | Greece | January 1, 2011 | — | — |
| OKeh | United States | 1918 | Imprint | — |
| OL Oil | Japan | April 2009 | Original Production | — |
| Olga Records | Sweden | January 1, 1964 | — | 1 |
| Om Records | — | 1995 | Original Production | — |
| Omnivore Recordings | United States | January 1, 2010 | — | 4 |
| Ondine | — | 1985 | Original Production | — |
| One Little Independent Records | United Kingdom | June 10, 2020 | Original Production | — |
| One Little Independent Records | United Kingdom | January 1, 1985 | — | 4 |
| One Little Indian Records | United Kingdom | 1985 | Original Production | — |
| Onimusic | Brazil | January 1, 2004 | — | 1 |
| Only the Family | United States | January 1, 2010 | — | 2 |
| OnlyMusic | Taiwan | July 11, 2013 | — | — |
| ONO Entertainment | South Korea | January 1, 2016 | — | — |
| Open World Entertainment | South Korea | January 1, 2000 | — | — |
| Opera Rara | United Kingdom | January 1, 1970 | — | — |
| Opus | Slovakia | January 1, 1971 | — | 3 |
| Orbis | United Kingdom | June 1, 1970 | Imprint | — |
| Orbis Publishing Ltd. | United Kingdom | January 1, 1970 | — | — |
| Oreade Music | Netherlands | January 1, 1986 | — | 1 |
| Orfeon | Mexico | January 1, 1958 | — | 1 |
| Orfeu | Portugal | January 1, 1956 | — | 5 |
| Oriente Musik | Germany | January 1, 1994 | — | — |
| Original Dub Gathering | France | 2005 | — | — |
| Original Jazz Classics | United States | 1983 | Reissue Production | — |
| Orlando | France | January 1, 1970 | — | 19 |
| Osmose Productions | — | 1991 | Original Production | — |
| Osmose Productions | France | January 1, 1991 | — | 3 |
| Oui Entertainment | South Korea | June 26, 2017 | — | — |
| Out of Line | — | 1995 | Original Production | — |
| Out of Line Music | Germany | January 1, 1995 | — | 9 |
| Outside Music | Canada | January 1, 2001 | — | 1 |
| Oveja Negra | Chile | January 1, 2006 | — | 1 |
| OVO Sound | Canada | January 1, 2012 | — | 2 |
| Owl Records | France | January 1, 1975 | — | 1 |