O

O: The symbol for diminished.

Oblique Motion: Two melodic lines, where one voice remains stationary, while the other moves.

Octave: An interval eight triads cale degrees above it. Two notes an octave apart have the same letter name, and form the most consonant interval possible.

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

Oeuvre: (Fr.) opus.

Offertory 1. A part of the Roman Catholic Mass proper, following the Credo and sung during the offering. 2. In Protestant church services, any music sung or played during offering.

Offices: Canonical Hours celebrated every day at stated times in the Catholic faith (i.e., Vespers, Matins). The Offices are one of two classes of services, whereas the other is the Mass. The Offices consist of prayers, antiphons, psalms, canticles, responses, hymns and readings. The principle music used in Offices are the chanting of psalms with their antiphons, the singing of hymns and canticles, and the chanting of lessons with their responses known as responsories.

Open Fifth: A triad that does not contain a third, consisting only of a root and a fifth.

Opera: A musical play, usually entirely sung, making use of costumes, staging, props, sets, and dramatic elements. Operas usually consist of two types of musical elements, the aria, which primarily expresses a single idea or theme, and the recitativewhich advances the story.

Opera Comique: comic opera.

Operetta: A light opera.

Opus: "Work". With a number, used to show the order in which the works by a given composer were written or published. Opus numbers are most often used for composers who catalogued their own works.

Oratorio: An operatic work without staging, sets, or elaborate costumes. Usually performed in a more relaxed setting than a formal opera, and usually having a religious theme.

Orchestration: The art of arranging, writing or scoring music for an 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.

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

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.