| Airline Name | ICAO | IATA | Callsign | Country |
| Oasis Hong Kong Airlines | OHK | O8 | OASIS | Hong Kong |
| Oasis International Airlines | AAN | OB | Spain | |
| Occitania Jet Fleet | OJF | France | ||
| Ocean Airlines | VCX | VC | OCEANCARGO | Italy |
| Oceanair | ONE | O6 | OCEANAIR | Brazil |
| Odessa Airlines | ODS | 5K | Ukraine | |
| Odette Airways | OAW | Switzerland | ||
| Odyssey International | ODY | OL | Canada | |
| Offline Officers | United Kingdom | |||
| Okada Airlines | OKJ | Nigeria | ||
| Okay Airways | BK | China | ||
| OLT | OLT | OL | Germany | |
| Olympic Air | OAL | OA | OLYMPIC | Greece |
| Olympic Airlines | OAL | OA | OLYMPIC | Greece |
| Olympic Airways | OAL | OA | Greece | |
| Olympic Aviation | OLY | ML | Greece | |
| Oman Air | OMA | WY | OMAN AIR | Oman |
| Oman Aviation | OAS | WY | Oman | |
| Omni Air International | X9 | United States | ||
| One-Two-Go | OX | Thailand | ||
| Ontario Express | OEL | 9X | Canada | |
| Onur Air | OHY | 8Q | Turkey | |
| OpenSkies | BOS | EC | MISTRAL | United Kingdom |
| Orange Air | ORJ | ORANGE SIERRA | Sierra Leone | |
| Orbest | OBS | Portugal | ||
| ORBI Georgian | Georgia | |||
| Orca Air | ORK | Egypt | ||
| Orel State Air Enterprise | ORM | Russian Federation | ||
| Orenburg Airlines | ORB | R2 | Russian Federation | |
| Orex Orbit Express Airlines | ORX | Turkey | ||
| Orient Eagle Airways | OEG | 4R | Kazakhstan | |
| Orient Thai Airlines | OEA | OX | Thailand | |
| Oriental Air Bridge | NGK | Japan | ||
| Origin Pacific Airways | OGN | QO | New Zealand | |
| Orion Air | TAG | HS | TAGGE | United States |
| Orion Airways | ORN | KG | United Kingdom | |
| Orionair | ORI | RED GLOBE | Spain | |
| Orix | United States | |||
| Our Airline | RON | ON | OUR AIRLINE | Nauru |
| Overland Airways | OLA | OJ | Nigeria | |
| Overseas National Airlines | OV | United States | ||
| Ozark Airlines | OZA | OZ | United States | |
| OzJet | OZJ | OZ | Australia |