Octave

The eighth tone above a given pitch, with twice as many vibrations per second, or below a given pitch, with half as many vibrations.

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Octet

A piece for eight instruments or voices.

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Open fifth

A triad without a third.

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Open strings

Strings are not stopped, fingured, or fretted.

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Opus, Op

The term, meaning work, is used by composers to show the chronological order of their works, e.g. Op. 1, Op. 2.

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Orchestra

A large group of musicians made up of string, brass, woodwind, and percussion instruments.

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Orchestration

The art of writing, arranging, or scoring for the orchestra.

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Ornamentation

Note or notes added to the original melodic line for embellishment and added interest.

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Ornaments

Melodic embellishments, either written or improvised.

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Ossia

"Or." Indicating an alternative passage or version.

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Ostinato

A repeated melodic or rhythmic pattern, frequently appearing in the bass line.

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Ottava

Octave.

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Ottava alta

(8va) An octave higher.

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Ottave bassa

(8va or 8vb) An octave lower.

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Overtones

The almost inaudible higher tones which occur with the fundamental tone. They are the result of the vibration of small sections of a string (instrument) or a column of air. Other general terms for overtones are partials and harmonics.

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Overture

The introductory music for an opera, oratorio or ballet. A concert overture is an independent work.