| O´Connor Mt Gambier´s Airline | UQ | OCM | |
| Oasis Hong Kong Airlines | O8 | OHK | |
| Occidental Airlines | OCT | ||
| Ocean Airlines | VC | VCX | |
| Okada Air | 9H | OKJ | |
| Okay Airways | BK | OKA | |
| OLT- Ostfriesische Lufttransport GmbH | OL | OLT | 704 |
| Olympic Airways | OA | OAL | 050 |
| Oman Air | WY | OAS | 910 |
| Omega Aero | |||
| Omni Air Express | X9 | OAE | |
| Omskavia | OMS | ||
| Onur Air | 8Q | OHY | |
| Orbi Georgian Airlines | NQ | DVU | 819 |
| Orenburg Airlines | R2 | ORB | 291 |
| Orex Airlines | ORX | 720 | |
| Orient Air | 7L | ORI | |
| Orient Thai Airlines | OX | OEA | 578 |
| Orient-Avia | ORT | ||
| Oriental Air Bridge | NGK | ||
| Oriental Airlines | OAC | ||
| Oriol Avia Soviet Austrian | OAU | 584 |