Publishers – O
- O.Ruby Productions
- O'Hardy Music
- Oak Archives
- Oak Publications
- OBH Musikverlag Otto B. Hartmann
- Obrasso Verlag
- Ocarina Workshop
- Occident Verlag
- Octava Music
- Ojot
- Oktavian Music
- Olaf Böhme
- Oldenbourg Verlag
- Oldenkamp-Kastelein Publications
- Olms
- Olympia Music Publishing
- Omni Music Publishing
- Omnibus Press
- One Little Independent Records
- Ongaku No Tomo Sha Corp.
- Open White Book
- Opera tres
- Opus 10
- Opwekking lectuur
- OR-TAV Music Publications
- Orania Music
- Oriana Music
- Oriel Library
- Orpheus Music
- Ossian Publications
- Otago Press
- OTON Musikverlag
- Otos Edizioni Musicali
- Otto Junne Musikverlag
- Otto Müller Verlag Salzburg
- Otto Wrede Regina-Verlag
- OUP Archive
- Out of the Ark Music
- Outline Press Bielo
- Ovation Press
- Oxford University Press
