- O.O. Discs
 - O2 Music Pty Ltd
- Record company based on the Internet features their publishing catalogue online and news from around the world.
- Obscured Records
- Features a fusion of obscure electronica, metal, and acoustic music. Artists include Anti-Melodic Chaotic Audio Assault Technicians, Angstophe, Fettucini Ink., Dead Obscured, and others.
- Ocean Inc.
- Independent hip-hop, R&B, and comedy label.
 Ochre Records
- October
- Home to DE/Vision and many more. Synth oriented pop with an industrial backbone.
- October Records
- Independent record label showcasing a new generation of imaginative Twin Cities recording artists.
- Oglio Records
- Specialises in complilations, new wave music and Jackie "the Joke Man" Martling.
- Oh Boy Records/Blue Plate Music/Red Pajamas Records
- Ojet Records
- Indie rock record label.
- Okka Disk
- Featuring free and improv jazz from around the world.
- OKra Records
- Om Records
- Spanning deep house, hip-hop, acid jazz, and other flavours of tasty beats. Artists include Marques Wyatt, Mark Farina, Soulstice, People Under the Stairs, and Mark Grant.
- Omnibus Records
- Indie-noise-pop-post-rock including releases by Maxiwagon, Skyscraper, Torpedo, Shove, Electro Group, Mates of State, and The Shins.
- Omnium Recordings
- Hard trad, a.k.a. world music that rocks or folk music under the influence.
- On The Rag
- Hardcore record label and zine.
- On-U Sound (2)
- See sites about the influential record label founded by London dub producer Adrian Sherwood.
- On/On Switch
- Punk label and distro featuring releases from Radar Secret Service, The Piranhas, and others.
- Oncus
- Sells designer T-shirts, as well as offering a record label, and more.
- One Little Indian
- One Way Productions
- Releases underground music by Boston punk and alternative groups including The Outlets, Shake The Faith, Left Nut, Last Stand, and others.
- One-Eleven Ltd.
- Recording label for violin enthusiasts.
- Ongaku Records, Inc.
- Features top soloists from around the world.
- Opening Day Recordings
- Canadian artists and composers.
- Opera Rara
- Reviving forgotten operas through CDs, anthologies, librettos, and illustrated notes.
- Operative Records
- Independent label producing industrial, noise, avant-garde and experimental alternative acts.
- Oracle Records
- Independent record label, distributor, and CD manufacturer based in Brisbane, Australia.
- Orange Recordings
- Indie rock, pop, underground, country, blues, and garage bands.
- Orange Twin
- Co-op web store and artists' gallery that raises funds to support wilderness preservation.
- Org
- The record label of the Organ, the long running U.K. fanzine.
- Origin Records
- Featuring New Stories, RadioAction, Don Lanphere and others.
- Original Cast Records
- Featuring rare Broadway, Off-Broadway, and cabaret artists.
- Orillyon Entertainment
- Representing the D.C./Baltimore sound.
- Ota Records
- Includes artist biographies, music samples, and CDs.
- Outcaste Recordings
- Underground Asian breakbeat, from sitar funkadelia to tabla-driven drum 'n' bass. Artists include Nitin Sawhney, Badmarsh, Shri, and others.
- Outersanctum Music
- Recording studio and label.
- Outsider Records
- Offering street-punk, oi, hardcore, etc.
- Outward Music Company
- Artists include Solenoid, Jesse James, Nudge, CNSE, Signaldrift, Metatron and others.
- Ovenguard Music
- Supporting artist's exploration of experimental electronics, cross-cut, ambient, and collage.
- Over the Top! Records
- Home to Toshi Iseda.
- Overcome Records
- Also comprising Mosh Bart Industries. Releases from Inside Conflict, Dragbody, Ananda, Vitality, and others.
- Overdressed Late Guy Productions
- Founded by Park Stickney.
- Overnight-Music
- We plan to focus on artists who we believe have something really worthwhile to share with the world.
- Overture Music
- Provides artists, gigs, news, and more.
- Ovum Recordings
- Releases music by Josh Wink, King Britt, Jamie Myerson and others.
- Oyate Records
- Native American music recording and production company.
- Oz Productions
- Mexican record label and distributor of everything related to heavy metal.
- OZMOZ
- Instrumental record company provokes human creativity.
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