Manufacturers
- O-Two Medical Technologies Inc.
- Oberheim
- Octave Electronics Inc.
- Odys
- Ohaus Corporation
- Okidata
- Olivetti
- Olson Electronics Inc.
- Oltronix Electronics
- Olympus
- Omega
- Omnitronic
- One for All
- Onkyo
- Optonica
- Orban
- Oriel
- Orion Research Incorporated
- Oritel
- Ortofon
- Osker Block
- Outlaw Audio
- Output Technology
