| O Tela Group | | |
| O'Herron Music | | |
| O-Right Records | | United States |
| O. Crow | | |
| O.M.G. | | France |
| O.U.T. Production | | Germany |
| Obfuscate Permiter Records | | Canada |
| Objekt Schutz Tapes | | Germany |
| Obreros Del Sonido | | Spain |
| Obscure Delight | | |
| Obscure Soundz | | Germany |
| Obsidian Tapes | | United Kingdom |
| Obskur Records | | Germany |
| Obsolete Music | | United States |
| OBUH Records | | Poland |
| Obvious Cassette Products | | United Kingdom |
| Obvious Products | | United Kingdom |
| Ochimancia | | |
| Octuris Records | | |
| Ode to Gravity | | United States |
| Offerta Speciale | | Italy |
| Office Records | | |
| OFN | | United Kingdom |
| Ohio | | |
| Ohne Label | | Germany |
| Ohoho Production Wabduwab Label | | Germany |
| Ohrbuch Verlag | | Austria |
| Ojas Music | | United States |
| Oje Tapes | | Germany |
| Oktron Produktions | | United States |
| Old Europa | | |
| Old Europa Cafe | | Italy |
| OMAHaHa | | United States |
| Omba Records | | |
| OMBAI | | |
| Omni Sonic | | |
| Omnipresent Records | | |
| On A Faim ! | | France |
| One Last Dream | | Germany |
| Only Tapes | | Italy |
| Onlytapes Records | | |
| Onomatopoeia | | |
| Ontario Electronic Music | | |
| Ooh Ohh Records | | |
| Ooh Ooh! Music | | United States |
| Opcion Sonica | | Mexico |
| Opel Records | | |
| Open Space Video | | |
| Open Wound | | |
| Operating Theatre Tapes | | United Kingdom |
| Optics | | |
| Optional Art Records | | |
| Opulence! | | |
| Opus Die Society | | Netherlands |
| Ora | | |
| Ora Pro Nobis | | |
| Oral Gestation Artefacts | | |
| Orcadia Machina | | |
| Order Ov Wolves | | |
| ORF Landesstudio Niederösterreich | | |
| ORF Rundfunk | | |
| Organart | | |
| Organic Mind Workshop | | |
| Organic Tapes | | France |
| Organik / Weser Label | | Germany |
| Orgastotron Music | | |
| original | | |
| Original Cast Cassette | | |
| Orogenetics | | |
| Orphanage | | |
| Orphanage Records | | United States |
| Orpheus Productions | | Germany |
| Ortega Y Cassette | | Spain |
| Out Loud Productions | | United States |
| Out of Depression | | Germany |
| Out of the Blue | | |
| Outworlder Records | | |
| Overdub | | |
| OX-XO Tapes | | |
| OZO Tape | | Netherlands |