* Obscuro
* Oi! - 1980s style of British punk rock
* Old school hip hop - generic term for hip hop music recorded before approximately 1989
* Old time country
* Old-time - archaic term for many different styles that were an outgrowth of Appalachian folk music and fed into country music
* Oldies
* Olonkho - Yakut epic songs
* Oltului
* Ompa - Music by the Kaizers Orchestra
* Omutibo
* Ondo
* On ikki muqam - Uyghur classical music suite in 12 parts
* Oom pah band
* Opera - theatrical performances in which all or most dialogue is sung with musical accompaniment
* Oratorical calypso
* Oratorio - similar to opera but without scenery, costumes or acting
* Orchestra - a large ensemble, especially one used to played European classical music
* Orchestre
* Organ trio - a style of jazz from the 1960s that blended blues and jazz (and later "soul jazz") and which was based around the sound of the Hammond organ
* Organic ambient - often acoustic ambient music which uses instruments and styles borrowed from world music
* Organic house
* Organica - A genre music created by SLIPS INTO SPACE in 2007, it is written without predetermining the outcome of the overall sound.This music causes audible hallucinations.
* Organum - Middle Ages polyphonic music
* Oriental Foxtrot
* Oriental metal - a subgenre of folk metal that incorporates elements of traditional Middle Eastern music.
* Orovela - eastern Georgian work songs
* Orgel (Organ Orgue) - keyboard instrument with/without pedals
* Orquestas Tejanas
* Ottava rima - Italian rhyming stanzas
* Outlaw country - late 1960s and 70s form of country music with a hard-edged sound and rebellious lyrics
* Outsider music - generic term for music performed by outsiders
* Özgün
* Ozwodna