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obbligato

Obbligato (Italian: obligatory) is often used virtually as a noun in English, in spite of its derivation. It is used to indicate an additional instrumental part that cannot be omitted, particularly when a solo instrument adds an accompanying melody in some baroque vocal forms. There is, for example, a well known violin obbligato to the mezzo-soprano aria Laudamus te, in the B minor Mass of Bach.

oblique motion

Two melodic lines where one moves while the other is stationary.

Oboe

The oboe is a double-reed instrument, an important part of the woodwind section of the modern orchestra. The mechanism of its keys underwent considerable development in the 19th century. In earlier times it formed an important part of the outdoor military band, but the Western symphony orchestra normally uses a pair of instruments. The oboe d'amore is the alto of the oboe family, used in the baroque period, and the tenor is found in the cor anglais or, in the mid-18th century, in the oboe da caccia. The tone of the instrument, much affected by different methods of cutting the reeds, can impart a characteristic sound to a whole orchestra.

octave

The interval between the first and eighth degrees of the diatonic scale.

octet

A piece written for eight parts, or the group that performs such a piece.

oeuvre

(Fr.) opus. Offertory A part of the Roman Catholic Mass (Proper), following the Credo and sung during the offering.

In Protestant church services, any music sung or played during offering.

ondes Martenot

The ondes Martenot, an electronic instrument invented by the French musician Maurice Martenot, produces single sounds by means of a keyboard that controls the frequencies from an oscillator. It has a wide range and offers the possibility of glissando. It became popular among French composers, including Milhaud, Honegger, Koechlin, Schmitt, Ibert, Jolivet, Messiaen and Boulez. Varèse also wrote for it, as he did for the less versatile electronic instrument, the theremin.

open fifth

A triad that does not contain a third.

opera

A dramatic work set to music in which all or most of the text is sung with instrumental accompaniment, costumes and sets.

opera bouffe

Opéra bouffe is the French term for comic operetta of composers such as Offenbach in 19th century France.

opera buffa

Opera buffa is Italian comic opera, particularly in the form it took in early 18th century Italy.

opéra comique

Comic Opera.

opera seria

Opera seria was the form of Italian serious opera that held sway from the reforms of the early 18th century for a hundred years. It came to be governed by strict rules as to subject and structure, and underwent reform in the interests of greater realism in the second half of the 18th century with the composer Gluck.

operetta

A light opera.

opus

“Work.” With a number, used to show the order in which pieces were written or published.

oratorio

A musical setting for voices and orchestra of a text based on the Scriptures or an epic theme. Essentially, an opera without staging, scenery or costumes.

orchestra

The orchestra, the dancing-place of the ancient Greek theatre, came, in the early 18th century, to have its modern meaning as a group of instrumental performers of varied number, although this meaning still met with objections at the time. The size and composition of the orchestra has differed from century to century, but during the course of the 17th century the string section developed as a five-part and later as a four-part section, with first and second violins, violas and cellos and double basses, the last two playing the same part, although the double basses would sound an octave lower. In the later 18th century it became usual to have in the orchestra an additional pair of French horns and a pair of oboes, doubling flute as necessary, with a bassoon doubling the bass. By the end of the 18th century a larger ensemble that included when necessary a pair of trumpets and drums was usual. In the 19th century clarinets, already used by Mozart and Haydn, became a regular part of the woodwind section, in addition to flutes, oboes and bassoons. The brass section came to include trombones, instruments earlier used for special purposes only, as well as trumpets, to be extended to instruments of lower range during the century. The 20th century has brought an extension of the percussion section. The number of players involved in a full symphony orchestra has grown very considerably, with over sixty string players, and a possible forty or more wind and percussion players. This compares with Mozart's Salzburg orchestra of 23 string players and a dozen or so wind-players and the orchestras of less prosperous princedoms, which might employ much smaller forces, a dozen or less string players and four or five wind players.

orchestration

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

Ordinary

In the Mass, the parts that are used every day, as distinct from the Proper. The Ordinary consists of the Kyrie, Gloria, Credo, Sanctus, and Agnus Dei.

organ

The organ is a keyboard instrument in which the sound is produced by air passing through pipes of various size and construction to give a wide variety of pitches and timbres. The instrument has its probable Western origin in the Hellenistic period, with the water-organ of Alexandria. Varying in size and mechanical efficiency, the organ had by the later 17th century given rise to an important school of performance, leading directly to the achievement of Johann Sebastian Bach in the first half of the 18th century. Technical developments have taken place since then, giving still greater versatility to the king of instruments.

ornament

A melodic embellishment, either written or improvised. Types of ornaments included trills, turns, mordents, and grace notes.

ostinato

Ostinato (Italian: obstinate) indicates a part that repeats the same rhythm or melodic element. The basso ostinato or ostinato bass occurs in the ground bass of baroque arias where a melody is set over a repeated bass pattern. Ostinato is used by the Bavarian composer Carl Orff in his instrumental teaching methods, where it may form a basis for improvisation by pupils.

overtone

A tone that is present in the sounding of a fundamental, due to the physics of the production of musical tones.

overture

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