Manufacturers:
- Oberheim
- Octave Electronics Inc.
- Odys
- Okidata
- Olivetti
- Olson Electronics Inc.
- Oltronix Electronics
- Olympia Werke AG
- Olympus
- Omega
- Omnitronic
- One for All
- Onkyo
- Optonica
- Orban
- Oriel
- Orion Research Incorporated
- Oritel
- Osker Block
- Output Technology
- OVP Orava
