Publishers – O
- O.Ruby Productions
- Oak Archives
- Oak Publications
- OBH Musikverlag Otto B. Hartmann
- Obrasso Verlag
- Ocarina Workshop
- Octava Music
- Ojot
- Oktavian Music
- Oldenkamp-Kastelein Publications
- Olympia Music Publishing
- Omni Music Publishing
- Omnibus Press
- One Little Independent Records
- Open White Book
- Opera tres
- Opus 10
- Opwekking lectuur
- OR-TAV Music Publications
- Orania Music
- Oriana Music
- Orpheus Music
- Ossian Publications
- Otago Press
- OTON Musikverlag
- Otto Junne Musikverlag
- Otto Wrede Regina-Verlag
- OUP Archive
- Out of the Ark Music
- Oxford University Press
