 | Oman Air | 1981 | 42 |
 | Onur Air | 1992 | 25 |
 | Okay Airways | 2004 | 19 |
 | Orient Thai Airlines Company Ltd. | 1992 | 18 |
 | Omni Air International | 1984 | 14 |
 | Olympic Air | 2009 | 14 |
| Overland Airways | 2001 | 9 |
 | Oman Gvmt | - | 4 |
 | OJSC "Tajik Air" | 1924 | 4 |
 | OJSC "Komiaviatrans" | 1998 | 4 |
 | Openskies | 2009 | 3 |
 | Oriental Air Bridge Co., Ltd. | - | 2 |
 | Orbest S.A. | 2007 | 2 |
| Olympus Airways | 2015 | 2 |
 | OOO "Atran " | 1942 | 1 |
| Omniflys | 1992 | 1 |
 | OJSC Air Kyrgyzstan | 1992 | 1 |
| Omni Air Transport | - | - |
| Open Skies Consultative Commission (OSCC) | - | - |
| Oceanic Air Traffic Control | - | - |
| On Air | - | - |
| Ostend Air College | - | - |
| Ohio State University Flight Education Division | - | - |
 | Ocean Airways | - | - |
| Oman Royal Flight | - | - |
| Olimp Air | - | - |
| Ornge Air | - | - |
| OHL Air Charterflug | - | - |
| Ocean Air | - | - |
| Out of the Blue Air Safaris | - | - |
| Oxley Aviation | - | - |
| Owenair | - | - |
| Operadora de Lineas Ejecutivas | - | - |
| Oxus Airways | - | - |
| Ocean Sky Aviation | - | - |
| Ordabasy | - | - |
| Omni - Aviacao E Tecnologia | - | - |
| Omega Aircompany | - | - |
| Oscar Jet | - | - |
| Opal Air | - | - |
| Orion Air Charter | - | - |
| Oulun Tilauslento | - | - |
| Odessa Aircraft Plant | - | - |
 | OAG | - | - |
| Opera Jet | - | - |
| Ocean Wings Commuter Service | - | - |
| Oklahoma Department of Public Safety, Troop (O) | - | - |
| Office Federal de Aviation Civile | - | - |
| Ordos General Aviation | - | - |
| Oslo Politidistrikt, Helikoptertjenesten | - | - |
| Osh Avia | - | - |
| Omsk Airlines | - | - |
| Orscom Tourist Installations Company | - | - |
| Operadora Llaminmex | - | - |
| Orel Air Enterprise | - | - |
| Open Flight | - | - |
| Oryx Jet | - | - |
| Operational Support Airlift Command (OSACOM) | - | - |
| Orient Airlines | - | - |
| Omega Air | - | - |
| On-Track Aviation | - | - |
| OAS Helicopters | - | - |
 | Orange Air Sierra Leone | - | - |
| ORO Taksi | - | - |
| Ok-Trans | - | - |
| Orion | - | - |
| OLT Express Germany | - | - |
 | Orange Air | - | - |
| Old Dominion Freight LT Lines | - | - |
| Onetime Airlines Zambia | - | - |
| Oasis Flight Malta | - | - |
| Oxford Aviation Academy | - | - |
| Olympic Broadcasting Services | - | - |
| OK Business Aircraft | - | - |
| Omniblu | - | - |
| Olimex | - | - |
| Oxygene Airways | - | - |
| Operadora Turistica Aurora | - | - |
 | Ord Air Charter | - | - |
| Originair | - | - |
 | Orenair | - | - |
| Operadora de Transportes Aereos | - | - |
| Osob Airlines | - | - |
| Oasis Air | - | - |
| Ogooue Air Cargo | - | - |
 | Open Joint Stock Company "Angara Airlines" | - | - |
| Oriental Airlines | - | - |
| One Airlines | - | - |
| Organizacoes Mambra | - | - |
| Orion-X | - | - |
| Operational Aviation Services | - | - |
| Orlan-2000 | - | - |
| Organization Not Allocated A Designator On An Exclusive Basis | - | - |
| OFEK | - | - |