O
- Oakland, Oakland Airmotive Company – United States
- Oberlerchner, Josef Oberlerchner Holzindustrie – Austria
- Octans Aircraft - Brazil
- Offpiste Limited, Dursley, Gloucestershire, United Kingdom
- OGMA, Indústria Aeronáutica de Portugal – Portugal
- Oldfield, Andrew Oldfield – United States
- Oldfield, Barney Oldfield Aircraft Company – United States
- Olympic Ultralights, Olympic Ultralights LLC – United States
- OMAC, OMAC Inc – United States
- OMF, Ostmecklenburgische Flugzeugbau GmbH – Germany
- OneAircraft, Celje, Slovenia
- One Aviation - Albuquerque, New Mexico, United States
- Omni, Omni Titan Corporation – United States
- Omni-Weld, Omni-Weld Inc – United States
- On Mark, On Mark Engineering Company – United States
- O'neill, O'Neill Airplane Company – United States
- Optica, Optica Industries Ltd – United Kingdom
- Option Air, Option Air Reno – United States
- Orel Aircraft, Selles-Saint-Denis, France
- Orlican, Orlican Národní Podnik – Czech Republic
- Oskbes-Mai, Otraslevoe Spetsialnoe Konstruktorskoe Byuro Eksperimentalnogo Samolyotostroeniya-Moskovskogo Aviatsionnogo Instituta – Russia
- Osprey, Osprey Aircraft – United States
- Österreichische-Ungarische Albatros Flugzeug Werke, Österreichische-Ungarische Albatros Flugzeug Werke – Austro-Hungarian Empire, (1914–1917) > Phoenix Flugzeugwerke
- Ottawa Car Manufacturing, Ottawa Car Manufacturing – Canada, (Ottawa Car Company, The Ottawa Car and Aircraft Company)
- Ozone Gliders, Le Bar-sur-Loup, France