Labels – O
- Oberlin Music
- Oboe Classics
- Obsculta Music
- Obsidian Records
- Ocora
- Octa Records
- Octatonic Records
- Ode Records
- Odradek Records
- Odyssey Opera
- Oehms
- Off
- Oktav
- Old Bridge Music
- Old Soul Music
- Olde Focus Recordings
- Olympia
- Olympic Records
- OMF
- Omnibus Classics
- OnClassical
- Ondine
- One Little Independent Records
- Onyx
- Onze Heures Onze
- Opening Day
- Opera dOro
- Opera Omnia
- Opera Rara
- Operation Seraphim
- Opus 3
- Opus Arte
- Opus Kura
- Opus Series
- Orange Mountain
- Orchid Classics
- Orfeo
- Organ Historical Society
- Organ Promotion
- Oriente Musik
- Origin Classical
- Origin Records
- Orlando Records
- Orpheus
- Orpheus Chamber Orchestra
- Orquestra Sinfonica Castilla y Leon
- OSJ Alive
- Ostinati Officina Musica
- Other Minds
- OUR Recordings
- Outhere
- Outnote Records
- Oxford Philomusica
- Oxford University Press Music
- OxRecs
- Ozella Music