| O. S. Services Audio Distribution | Audion and Cadence Tube Amplifiers |
| OAO | Tubes |
| Ocean Acoustic loudspeakers | Speakers |
| Octave | Tube Amplifiers |
| Octave Electronics | Parts |
| Odyssey Pro Sound | Pro Audio Recording Equipment |
| Odyssey Pro Sound | Retailers |
| OEI Obsolete Electronics Inc. | Information about Tube Audio, Transformers |
| Official The-Valve | Tube Guitar Amplifiers |
| Oguri | DIY Tube Amplifiers |
| Oh's Audio | Tube Amplifiers, Tube Amplifiers Kits, Forum, Parts |
| OKKUN_Audio_Tube_Amplifire | Japanese Tube Amplifiers |
| Okmusic | Retailer |
| Old Radio | Tube Radios & Related Vintage Items |
| Old Time Radio | Für Sammler und Liebhaber alter Radiogeräte |
| Old timers Nostalgia page | Associations, Links |
| Old Tube Radio (Vintage Radio), Luxembourg | Tube Radios & Related Vintage Items |
| oldtube.com | Tubes |
| Olga Tube Amplifiers | Tube Amplifiers |
| OlimpiaAudio | Tube Amplifiers, Tubes, Transformers, Parts |
| Olionik | Tube Amplifiers |
| Omak | Tube Amplifiers |
| Omega Speaker Systems | Speakers |
| Omicron | Tube Preamplifier |
| One Electron | Output Transformers |
| Onkel frihu's Community das Forum | German Forum about Tube Audio |
| Opera Audio | Tube Amplifiers |
| Opera Audio Germany | Tube Amplifiers, Speakers |
| Oppermann Electronic | Parts |
| Orator | Tube Amplifiers, Speakers |
| Orcadesign | speakers |
| ORCHID PRECISION AUDIO | Tube Amplifiers, Speakers |
| Oren Elliott Products | Air dielectric capacitors, inductors |
| Organized Services | Tubes |
| Orvac Electronics | Tubes, Parts |
| Osborn Loudspeakers | Speakers, |
| Oskari Heinonen | Links |
| OUÏR Lautsprecher der Spitzenklasse | Speakers, Retailer |
| Outline of JAS | Japan Audio Society |
| Overkill Audio | Speakers |