Airline codes| IATA | ICAO | Airline | Call sign | Country | Comments |
|---|
| ORN | Orange Air | ORANGE JET | United States | Allocated in 2014[1] |
| IP | ONS | One Airlines | AIR DREAMS | Chile | |
| FET | Old Dominion Freight Lines | FREIGHT LINE | United States | |
| OCN | O Air | O-BIRD | France | defunct |
| UQ | OCM | O'Connor Airlines | OCONNOR | Australia | Defunct - Bankrupt |
| DRL | Omni Air Transport | DRILLER | United States | |
| OWE | Owenair | OWENAIR | South Africa | |
| OB | AAN | Oasis Airlines | Oasis | Spain | defunct |
| O8 | OHK | Oasis Hong Kong Airlines | OASIS | Hong Kong | Defunct |
| BCN | Ocean Air | BLUE OCEAN | Mauritania | |
| VC | VCX | Ocean Airlines | OCEANCARGO | Italy | |
| OCS | Ocean Sky UK | OCEANSKY | United Kingdom | |
| TUK | Ocean Wings Commuter Service | TUCKERNUCK | United States | New Island Connections |
| O2 | | Linear Air | HARPOON | United States | Allocated in 2017 |
| ODS | Odessa Airlines | ODESSA AIR | Ukraine | |
| FOC | Office Federal De'Aviation Civile | FOCA | Switzerland | |
| GBO | Ogooue Air Cargo | | Gabon | |
| OKJ | Okada Airlines | OKADA AIR | Nigeria | |
| OKP | Okapi Airways | OKAPI | Democratic Republic of Congo | |
| OKA | Okay Airways | OKAYJET | China | |
| OKL | Oklahoma Department of Public Safety | OKLAHOMA | United States | Troop O |
| OLX | Olimex Aerotaxi | OLIMEX | Czech Republic | |
| KVK | Olimp Air | PONTA | Kazakhstan | |
| OL | OLT | OLT Express Germany | OLTRA | Germany | |
| OA | OAL | Olympic Air | OLYMPIC | Greece | |
| OLY | Olympic Aviation | OLAVIA | Greece | |
| WY | OMA | Oman Air | OMAN AIR | Oman | |
| ORF | Oman Royal Flight | OMAN | Oman | |
| OV | OMS | SalamAir | MAZOON | Oman | Oman’s first Low Cost Carrier |
| OAV | Omni - Aviacao e Tecnologia | OMNI | Portugal | |
| OY | OAE | Omni Air International | OMNI-EXPRESS | United States | |
| ONI | OMNI AVIATION TRAINING CENTER | OMNI TRAINING | Portugal | Part of the same group as Omni - Aviacao e Tecnologia |
| OMF | Omniflys | OMNIFLYS | Mexico | |
| ORL | On Air Limited | ON AIR | Canada | |
| OST | Oklahoma State University | OSTATE | United States | |
| OTG | One Two Go Airlines | THAI EXPRESS | Thailand | |
| OTM | Onetime Airlines Zambia | ZEDTIME | Zambia | |
| MED | Ontario Ministry of Health | MEDICAL | Canada | |
| 8Q | OHY | Onur Air | ONUR AIR | Turkey | |
| OPA | Opal Air | | Australia | |
| OSA | Open Sky Aviation | | Lebanon | |
| BOS | OpenSkies | MISTRAL | United Kingdom | |
| ORR | Operadora Turistica Aurora | TURISTICA AURORA | Mexico | |
| OLE | Operadora de Lineas Ejecutivas | OPERADORA | Mexico | |
| OTP | Operadora de Transportes Aéreos | OPERADORA AEREO | Mexico | |
| OPV | Operadora de Veulos Ejectutivos | OPERADORA DE VUELOS | Mexico | |
| LLO | Operation Enduring Freedom | APOLLO | Canada | |
| OAX | Operational Aviation Services | | Australia | |
| ORD | Orange Air Services | ORANGE SERVICES | Sierra Leone | |
| ORJ | Orange Air Sierra Leone | ORANGE SIERRA | Sierra Leone | |
| ORE | Orange Aviation | ORANGE AVIATION | Israel | |
| ORX | Orbit Express Airlines | OREX | Turkey | |
| ORK | Orca Air | ORCA TAXI | Egypt | |
| BUE | Orebro Aviation | BLUELIGHT | Sweden | |
| ORM | Orel State Air Enterprise | ORPRISE | Russia | |
| R2 | ORB | Orenburg Airlines | ORENBURG | Russia | |
| OTA | Organizacion De Transportes Aéreos | ORGANIZACION | Mexico | |
| OML | Organizacoes Mambra | MAMBRA | Angola | |
| OVV | Orient Air | ORIENTSYR | Syrian Arab Republic | |
| OTR | Orient Airlines | ORIENTROC | Sudan | |
| ORN | Orient Airways | ORIENT LINER | Pakistan | |
| OX | OEA | Orient Thai Airlines | ORIENT THAI | Thailand | |
| NGK | Oriental Air Bridge | ORIENTAL BRIDGE | Japan | |
| OAC | Oriental Airlines | ORIENTAL AIR | Nigeria | |
| QO | OGN | Origin Pacific Airways | ORIGIN | New Zealand | |
| OED | Orion Air Charter | ORION CHARTER | South Africa | |
| OIX | Orion-x | ORIONIX | Russia | |
| KOV | Orlan-2000 | ORLAN | Kazakhstan | |
| RNG | Orange Aircraft Leasing | ORANGE | Netherlands | |
| OAD | Orscom Tourist Installations Company | ORSCOM | Egypt | |
| JPA | OSACOM | J-PAT | United States | United States Army |
| OSH | Osh Avia | OSH AVIA | Kyrgyzstan | |
| OCO | Ostend Air College | AIR COLLEGE | Belgium | |
| ODY | Odyssey International | ODYSSEY | Canada | |
| FNL | Oulun Tilauslento | FINN FLIGHT | Finland | |
| ON | RON | Our Airline | OUR AIRLINE | Nauru | formerly Air Nauru |
| OOT | Out Of The Blue Air Safaris | OOTBAS | South Africa | |
| OJ | OLA | Overland Airways | OVERLAND | Nigeria | |
| OAR | ONE AIR | BOSS AIR | Spain | |
| OXE | Oxaero | OXOE | United Kingdom | |
| WDK | Oxford Air Services | WOODSTOCK | United Kingdom | |
| OAA | Oxley Aviation | | Australia | |
| OZ | OZR | Ozark Air Lines | OZARK | United States | Defunct |
| O7 | OZJ | OzJet | AUSJET | Australia | |
| OSU | Ohio State University | SCARLET | United States | |
| OA | OAL | Olympic Airlines | OLYMPIC | Greece | Defunct |
| OB | AAN | Oasis International Airlines Now assigned to Boliviana de Aviacion (BoA) | OASIS | Spain | Ceased operations |
| ORT | Ortac | SKYWALKER | Jersey | | |
