Glossary of Musical Terms


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.


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 octave is an interval of an eighth, as for example from the note C to C or D to D. The first note can have a sharp or flat providing the last note has the corresponding sharp or flat (i. e. C sharp to C sharp).


An octet is a composition for eight performers.

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.


An opera is a drama in which most of the actors sing all or most of their parts. The form developed at the end of the 16th century in Italy, from where it spread to other regions of Europe, although it never became a regular part of London musical life until the early 18th century. Internationally Italian opera has proved immensely important and popular, while opera in France underwent independent development in the later 17th century under the Italian-born composer Lully.

Opéra bouffe

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

Opéra bouffa

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

Opéra comique

French opéra comique originally purely comic and later more sentimental in mood, included spoken dialogue, interspersed with songs.

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 is light opera, a development largely of the 19th century, exemplified in the work of Offenbach in France and Johann Strauss the younger in Vienna.


Opus (= Latin: work) is generally used in the listing of a composer's works by opus numbers, usually abbreviated to Op. Since the Latin plural opera would lead to unnecessary confusion it is best avoided, although the alternative opuses remains an unsatisfactory substitute. Opus numbers are not always a guide to the date of composition or even to the date of publication.


Oratorio has its origin in the musical performances used by the followers of St. Philip Neri, the Oratorians, a religious order founded in 1575, although it has a possible remoter origin in the liturgical drama of the Middle Ages. Forms of oratorio change, but it remains primarily a work in which religious texts often with a narrative content are set for performance by singers and instruments.


A chamber orchestra has come to indicate an orchestra smaller in size than the usual symphony orchestra.


Orchestration is the art of arranging music for the orchestra or the way in which this is done.


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.


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.


The overture (= French: ouverture; German: Ouvertüre; Italian: sinfonia) is an introductory piece, often designed to initiate an opera or other dramatic work. The late 17th century French overture of Lully opens with a slow section in dotted (uneven) rhythm, followed by a fugal section, before the return of the slow opening. The Italian overture provides the origin of the symphony, with two fast movements framing a central slow movement. The word Ouvertüre or Ouverture is sometimes used to mean an orchestral suite, as in the four orchestral suites of Johann Sebastian Bach.