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.

Octet - A piece for eight instruments or voices.

Open fifth - A triad without a third.

Open strings - Strings are not stopped, fingured, or fretted.

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

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

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

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

Ornaments - Melodic embellishments, either written or improvised.

Ossia - "Or." Indicating an alternative passage or version.

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

Ottava - Octave.

Ottava alta - (8va) An octave higher.

Ottave bassa - (8va or 8vb) An octave lower.

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.

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