O: The symbol for diminished.
Oblique Motion: Two melodic lines, where one voice remains stationary, while the other moves.
Octet: A piece written for eight parts, or the group that performs such a piece.
Oeuvre: (Fr.) opus.
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 recitative which 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.
Overtone: A tone that is present in the sounding of a fundamental, due to the physics of the production of musical tones.