| Term | Description |
|---|
| Obbligato | In classical music obbligato usually describes a musical line that is in some way indispensable in performance. Its opposite is the marking ad libitum. |
| Octave | In music, an octave (sometimes abbreviated 8ve or P8) is the interval between one musical pitch and another with half or double its frequency. |
| Octet | In music, an octet is a musical ensemble consisting of eight instruments or voices, or a musical composition written for such an ensemble. |
| Open, closed, open | Open, closed, open is a technique of playing snare drum rudiments, especially used during auditions. |
| Open form | In music, open form refers to Aleatoric music. |
| Opera | Opera is a form of musical and dramatic work in which singers convey the drama. |
| Opus | In music, pieces of music by composers are given opus numbers, which generally run in order of publication. Unpublished compositions often are given WoO numbers. |
| Oratorio | An oratorio is a large musical composition including an orchestra, a choir, and soloists. The oratorio was somewhat modeled after the opera. |
| Orchestra | An orchestra is an instrumental ensemble, usually fairly large with string, brass, woodwind sections, and possibly a percussion section as well. |
| Orchestral percussion | Orchestral percussion are percussion instruments used in classical music. |
| Orchestration | Orchestration is the study or practice of writing music for orchestra (or, more loosely, for any musical ensemble) or of adapting for orchestra music composed for another medium. It only gradually over the course of music history came to be regarded as a compositional art in itself. |
| Organ stop | An organ stop (or just stop) is a component of a pipe organ which admits pressurized air (known as wind) to a set of organ pipes. Its name comes from the fact that stops can be used selectively by the organist; some can be "on" (admitting the passage of air to certain pipes), while other can be "off". |
| Organology | Organology is the science of musical instruments and their classification. It embraces study of instruments' history, instruments used in different cultures, technical aspects of how instruments produce sound, and musical instrument classification. |
| Ornament | In music, ornaments are musical flourishes that are not necessary to the overall melodic (or harmonic) line, but serve to decorate or "ornament" that line. Many ornaments are performed as "fast notes" around a central note. |
| Ossia | Ossia is a musical term for an alternate passage which may be played instead of the original passage. |
| Ostinato | In music, an ostinato (derived from Italian: "stubborn", compare English: obstinate) is a motif or phrase which is persistently repeated at the same pitch. |
| Outro | An outro (sometimes "outtro") or extro is the conclusion to a piece of music, literature or television program. It is the opposite of an intro. |
| Overblowing | Overblowing is a technique used in playing a wind instrument to produce a different pitch by changing the direction and/or force of the air stream. Overblowing can be done deliberately in order to get a higher pitch, or inadvertently, resulting in the instrument producing a note other than the one intended. |
| Overtone singing | Overtone singing, also known as throat singing, overtone chanting, or harmonic singing, is a type of singing in which the singer manipulates the harmonic resonances (or formants) created as air travels from the lungs, past the vocal folds, and out the lips to produce a melody. |
| Overture | Overture (French ouverture, meaning opening) in music is the instrumental introduction to a dramatic, choral or, occasionally, instrumental composition. |
