| OAA | VQ | 253 | Impulse Airlines |
| OAC | Oriental Airlines | ||
| OAE | X9 | Omni Air Express | |
| OAL | OA | 050 | Olympic Airways |
| OAS | WY | 910 | Oman Air |
| OAU | 584 | Oriol Avia Soviet Austrian | |
| OBM | Aviaobshemash | ||
| OCA | R7 | 717 | ASERCA Airlines |
| OCM | UQ | O´Connor Mt Gambier´s Airline | |
| OCT | Occidental Airlines | ||
| ODS | 5K | 654 | State Air Company Odessa Airlines |
| OEA | OX | 578 | Orient Thai Airlines |
| OGJ | Bakoji Airlines Services Ltd | ||
| OHK | O8 | Oasis Hong Kong Airlines | |
| OHY | 8Q | Onur Air | |
| OKA | BK | Okay Airways | |
| OKJ | 9H | Okada Air | |
| OLT | OL | 704 | OLT- Ostfriesische Lufttransport GmbH |
| OMS | Omskavia | ||
| ONT | GX | Air Ontario | |
| OOM | Zoom Airlines | ||
| ORB | R2 | 291 | Orenburg Airlines |
| ORI | 7L | Orient Air | |
| ORT | Orient-Avia | ||
| ORX | 720 | Orex Airlines | |
| OSL | Sosoliso Airlines | ||
| OTL | YG | South Airlines | |
| OWS | OW | Cargosur | |
| OXO | Millon Airlines | ||
| OZW | XR | 674 | Skywest Airlines |