Obscuro
Ocean Grunge Internet-born electronic grunge style mixed with oceanic sounds and ambiguous reverberated vocals
Oi! – 1980s style of British punk rock
Old school hip hop – generic term for hip hop music recorded before approximately 1989
Old-time – archaic term for many different styles that were an outgrowth of Appalachian folk music and fed into country music
Oldies – Popular music from the 50's and 60's.
Olonkho – Yakut epic songs
Old Time Radio – Old Time Radio Shows from the 30's – 60's
Ondo
Opera – theatrical performances in which all or most dialogue is sung with musical accompaniment
Operatic Pop – subgenre of pop music that is performed in a classical operatic style (also referred to as "Popera")
Oratorio – similar to opera but without scenery, costumes or acting
Orchestra – a large ensemble, especially one used to play European classical music
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
Organum – Middle Ages polyphonic music
Oriental metal – a subgenre of folk metal that incorporates elements of traditional Middle Eastern music.
Ottava rima – Italian rhyming stanzas
Ottoman military band – the oldest variety of military marching band in the world.
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
