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Obbligato -
A solo part which accompanies a main melody. Though intended to sound as if improvised, it is fully composed and so 'obligatory,' thus the name.
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Oboe da caccia -
An oboe a perfect fifth lower than the usual oboe. Play C and out comes the F below. In this respect it is like the modern english horn. The most significant difference is found in the shape of the interior cylinder (the bore).
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Oboe d'amore -
An oboe a minor third lower than the usual oboe, so a written C comes out as an A. Though most associated with baroque music, Ravel uses it from bar 77 onward in Bolero, and Richard Strauss uses it in his Sinfonia Domestica.
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Octave doubling -
Two instruments playing the same thing an octave apart.
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Octave -
The distance from one letter name of pitch to the next pitch with the same letter name. The relationship of the lower pitch to the higher is twice the vibrations. Thus, A below middle C is 220 hz, the A above middle C is 440 hz, and the A above the treble staff is 880 hz.
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'Ode' tune -
This is the most famous melody in Beethoven's Ninth.
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Off the string -
Obviously not literally off the string, but with a lightness of bowing that removes most of the pressure and has an upward impulse.
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Ondes martinot -
A melodic electronic instrument invented by Maurice Martenot in the 1920s. It is capable of changing both pitch and tone quality. One of its features is the kind of sliding between pitches that one hears in, say, the theme to the original Star Trek doubling the wordless soprano. It is a still used precursor to the modern synthesizer.
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One-holed trumpet -
Was one of several ways of trying to play more pitches than the ones granted by physics. Now we use a series of valves which lengthen the tube.
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Op. 65 bis -
"Bis" mean "again." Op. 65 was originally a set of works for young pianists. Since the piece in question is an orchestration of the same pieces, Op. 65 is merely done again - bis.
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Open C string -
The lowest string on the viola and cello. Open means no finger is pressing on the fingerboard.
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Open intervals -
The distances of the first, second, and third overtones. An octave, a fifth, and a fourth. They get that name because they could contain other notes which would make a chord, but in this case do not. They often have a hollow sound like that of the stereotypical monks chanting on two pitches. In early music the intervals had the religious significance of representing the Trinity: octave = God the Father; fifth = God the Son; fourth = God the Holy Spirit. ________________________
Open score -
One staff per part, no longer suggesting a keyboard performance.
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Open tone -
The sound generated from an unaltered string or column of air. This includes the playing of the *overtones present.
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Open tones (brass) -
The natural tones: fundamental for the length of the tube, octave higher, fifth higher from that pitch, fourth higher, major third, minor third, between major and minor third, between major and minor second (to reach the third octave from the fundamental), major second, etc. It is from these open tones that the valves of modern trumpets, horns, euphoniums, and tubas drop the sound. The trombone and the sackbut before it have always been able to extend to pitches below the open tones by the use of the slide.
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Opera buffa -
Comic opera. Originally using comedia dell'arte stereotypes. While this fits Rossini's Barber of Seville, and to a certain extent Mozart's Marriage of Figaro, it does not fit Don Giovanni, which is termed a dramma giocoso.
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Opera Comique style -
Uses spoken dialogue. Sung dialogue appeared in Grand Opera. No matter how serious the subject matter, the use of spoken dialogue branded it as Comique.

To place Carmen in an opera house, sung recitatives were added by Ernest Guiraud after Bizet's death.
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Opera seria -
Serious opera of the 17th and 18th centuries. These were stylized stories from mythology, romance, or classical history. Heroic male roles were sung by women or castrati. Almost every aria was a static contemplation of the character's current situation in the plot. In the face of much less stylized productions opera seria died out, though some consider Berlioz' Les Troyens to be one enormous opera seria in intent.
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Ophicleide -
A large brass instrument of the 19th century. It is a precursor of the tuba.
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Opp. -
The abbreviation for Opera, the plural of Opus, work.
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Ordinary (of the mass) -
Kyrie, Gloria, Credo, Osanna, Sanctus, Agnus Dei. Those portions of the mass text which remain the same each day.
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Organ -
Really nothing more than a "chest of whistles," as it was called by those objecting to instruments in the church. A box has air pumped into it by bellows and the air is released through whistles controlled by a player sitting at a keyboard. The ramifications of this simple definition are, of course, enormous.
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Organum -
An outgrowth of monophonic chant. In simple parallel organum a chant melody begins and the upper voice rises stepwise above it until it is a 5th higher (See: *Fifth). At that point it duplicates the chant exactly at the new pitch, thus running parallel with the original chant pitches. It is this early form of two-voiced writing which is so often used in ads and films to indicate a Roman Catholic church or a monastery.
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Original -
The first statement of a motive or theme, the way it is first heard. See also *Retrograde, *Inversion, and *Retrograde Inversion.
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Ornamentation -
Adding extra notes in conjunction with the written pitches. This practice flourished in classical music during the 18th century and in opera during the *bel canto period. It continues to flourish today in Gospel singing.
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Ostinato -
A short repetitive figure, usually in the bass, which continues while other material is used.
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Ottavio -
The lead tenor role in Mozart's Don Giovanni.
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Oud -
A lute of Arabic or Levantine origin.
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Overtones -
The same as *Harmonics, though that term is most often associated with string instrument playing. Overtones are the higher vibrations within a lower 'fundamental' vibration. The series has a constant relationship: Fundamental, octave up, fifth up, fourth up (second octave), major third up, minor third up, small minor third up, second up, and that brings us to the third octave above the fundamental. The series continues into gradations of whole and half steps that only brass players use regularly.