1. Oboe: A double-reed woodwind instrument with a distinctive high-pitched, nasal sound.
2. Organ: A keyboard instrument that produces sound by driving pressurized air through pipes of various lengths and shapes.
3. Ondes Martenot: An early electronic musical instrument that uses a theremin-like system to control pitch and timbre.
4. Ophicleide: An obsolete brass instrument with a distinctive conical shape and a large bell, resembling a bass clarinet.
5. Orff instruments: A collection of percussion instruments often used in music education, named after the German composer Carl Orff.
6. Ocarina: A small, flute-like wind instrument with a closed shape and finger holes, often made of clay or ceramic.