Publishers – O
- O.Ruby Productions
- Oak Archives
- Oak Publications
- Obrasso Verlag
- Ocarina Workshop
- Oertel
- Old Time Music and Dance
- Oldenkamp-Kastelein Publications
- Omnibus Press
- Onyx Brass
- Onze Music
- Opwekking lectuur
- Oriana Music
- Orpheus Music
- Ossian Publications
- Otago Press
- Otago University Press
- Otto Junne Musikverlag
- Otto Wrede Regina-Verlag
- OUP Archive
- Outline Press
- Overscore
- Oxford University Press