- Ô! Kosmos! (1973)chorus and soprano
- Ô Bali (1989), 11:002 flutes, vibraphone, piano, 2 violins, cello and double bass
- Ô Bali (1989), 11:00Arr. José Evangelista, 2000orchestraOriginal work: Ô Bali
- Ô Bali (1989), 11:00Arr. Alexandre David, 2017gamelanOriginal work: Ô Bali
- O cameretta, 3:00
- Ô Gamelan (2013), 16:00
- Ô Java (1993), 12:002 flutes, 2 oboes, 2 clarinets, 2 bassoons, 2 horns, 2 trumpets, trombone, piano, harp, 2 percussions and string instrument
- O Jesu mi dulcissime (1615)
- O Magnum Mysterium In Memoriam Glenn Gould (1984), 17:00string orchestra
- O Magnum Mysterium In Memoriam Glenn Gould (1984), 14:00Arr. John Rea, 1999string orchestraOriginal work: O Magnum Mysterium In Memoriam Glenn Gould
- O Mensch! (Inventaire raisonné de quelques passions Nietzschéennes) (2008-09), 70:00
- O mon amour, 1:55
- O passi sparsi, 3:00
- O Qin (2009), 6:00guqin, yangqin and orchestra
- O Sacrum convivium (1937)mixed chorus
- O Salutaris (1992)mezzo-soprano and organ
- O so sorry (1982), 3:00
- O Virtus (2018)mezzo-soprano, flute, piano and percussion
- Ô Yasmina (2007), 5:53oriental tenor, nay (or flute) and chamber orchestra
- Oaristys (1991), 14:36stereo fixed medium
- Objets perdus (1991), 18:00string quartet
- Obsession (1973)3 flutes, oboe, clarinet, bassoon, horn, piano, 2 percussions, 2 violins, 2 violas and 2 cellos
- L’Océantume (2009), 90:00mezzo-soprano, piano and saxophone quartet
- Octandre (1923)flute, oboe, clarinet, bassoon, horn, trumpet, trombone and double bass
- Octet (1979)string quartet, 2 flutes (piccolos), 2 clarinets (bass clarinets) and 2 pianos
- Octet (1996)flute, oboe, clarinet, piano, percussion, violin, cello and double bass
