| O, Od | (Italian) or |
| Obbligato (sometimes spelt incorrectly as Obligato) (It.), Obligé (Fr.), Obligat (Ger.) | literally 'necessary' or 'obligated', but frequently used to mean the reverse, i.e. 'may be omitted' or 'optional' |
| Oben | (German) above |
| Ober | (German) above |
| Oberek | a Polish dance |
| Obertas, Obertass | a quick (even wild), triple time, round dance from Poland |
| Oberwerk, Obw. (abbreviated form) | (German) Swell organ |
| Oboe | a family of double-reed wind-instruments, that produce a somewhat plantive tone |
| Oboe da Caccia | the precursor of the cor anglais |
| Oboe d'amore | lying between the standard oboe and the cor anglais the oboe d'amore is now obsolete |
| Ocarina | an egg-shaped, metal, plastic or earthenware instrument with fingerholes which produces its sound in a similar way to the recorder |
| Octave | sometimes abbreviated to 8ve |
| Octave flute | piccolo |
| Octet (Eng.), Octuor or Octette (Fr.), Ottetto (It.), Oktett (Ger.) | a work written for eight players, the group playing such a piece of music |
| Ode | originally a moderate length poem of praise, now a piece of music fulfilling a similar purpose |
| Oder | (German) or |
| Oeuvre | (French) work |
| Offen | (German) open |
| Öffnen | (German) to open |
| Ogni | (Italian) all, every |
| Ohne | (German) without |
| Ole | a Gypsy dance like a Spanish Seguidilla |
| Ondeggioando, Ondeggiante, Ondeggiamento | (Italian) tremolo, vibrato, undulating, a swaying effect (rhythmic) |
| Ondes martenot | an electronic musical instrument used often in music written by Messian |
| Ondulè | (French) tremolo, vibrato, undulating, a swaying effect (rhythmic) |
| Ongarese | (Italian) Hungarian |
| Oper | (German) opera |
| Operetta | a short opera, light opera |
| Ophicleide | a large member of the bugle family |
| Opus, Op. (abbreviated singular), Opp. (abbreviated plural) | (Latin) a musical work |
| Orageux, Orageuse | (French) stormy |
| Oratorio | a musical work set to a religious text for orchestra, choir and solo singers |
| Ordinaire (Fr.), Ordinario (It.) | ordinary, normal |
| Ordre | a term used by certain French composers of the baroque period to mean 'suite' |
| Organistrum | hurdy-gurdy |
| Organo | (Italian) organ |
| Orgel | (German) organ |
| Orgia | (Italian) orgy |
| Orgue | (French) organ |
| Orgue expressif | (French) harmonium |
| Orgue plein | (French) full organ |
| Orpharion, Orphareon | a member of the cittern family |
| Osservanza | (Italian) observation |
| Ossia | (Italian) an alternative (not necessarily easier) to the original passage |
| Ostinato | (Italian) persistent, as in 'basso ostinato' meaning 'ground bass' |
| Ôter | (French) to take off, as in 'Ôtez les sourdines' meaning' take off the mutes' |
| Ottava, 8va (abbreviated form) | (Italian) octave |
| Otto | (Italian) eight |
| Ottone | (Italian) brass |
| Ou, Où | (French) or, where |
| Ouvert, Ouverte | (French) open |
| Ouverture | (French) overture |
| Ouvrir | (French) to open |
| Overblowing | blowing or tonguing more strongly (generally while venting the thumbhole or opening a speaker key to the purpose) to produce the higher notes oncertain wind-instruments, e.g., on the recorder (which over-blows at the octave) or on the clarinet (which over-blows at the twelfth) |
| Overtones | harmonics |
| Overture | a piece that acts as an introduction to an oratorio or opera, a concerted work similar to a suite (i.e. having a number of movements) of two main types - the Italian (three movements, quick, slow, quick) - and the French (three movements or sections, slow, quick, slow) which, if sectional, might form the opening movement of a larger concerto |
| Ovvero | (Italian) or |