* Ukulele: A small, four-stringed guitar that is popular in Hawaii. It is often used for playing popular and folk music.
* Ondes Martenot: An electronic musical instrument that was invented in France in the 1920s. It is a single-keyboard instrument that produces a continuous, ethereal sound.
* Ophicleide: A large, brass instrument that is similar to a tuba. It was popular in the 19th century, but is now rarely used.