Airline codes| IATA | ICAO | Airline | Call sign | Country | Comments |
|---|
| ORN | Orange Air | ORANGE JET | United States | Allocated in 2014[1] |
| ONS | One Airlines | AIR DREAMS | Chile |
| FET | Old Dominion Freight Lines | FREIGHT LINE | United States |
| OCN | O Air | O-BIRD | France |
| UQ | OCM | O'Connor Airlines | OCONNOR | Australia | Defunct - Bankrupt |
| DRL | Omni Air Transport | DRILLER | United States |
| OWE | Owenair | OWENAIR | South Africa |
| JPA | OSACOM | J-PAT | United States | United States Army |
| 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 |
| O6 | ONE | Avianca Brazil | OCEANAIR | Brazil | formerly Oceanair |
| O2 | | Linear Air | HARPOON | United States | Allocated in 2017 |
| ODS | Odessa Airlines | ODESSA AIR | Ukraine |
| ODY | Odyssey International | ODYSSEY | Canada |
| 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 |
| 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 |
| OE | LDM | Laudamotion | LAUDA MOTION | Austria |
| OY | OAE | Omni Air International | OMNI-EXPRESS | United States |
| OMF | Omniflys | OMNIFLYS | Mexico |
| ORL | On Air Limited | ON AIR | Canada |
| 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 |
| OSH | Osh Avia | OSH AVIA | Kyrgyzstan |
| OCO | Ostend Air College | AIR COLLEGE | Belgium |
| OL | OLT | OLT Express Germany | OLTRA | Germany |
| 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 Airlines | AUSJET | Australia |
| OA | OAL | Olympic Airlines | OLYMPIC | Greece | Defunct |
| OB | AAN | Oasis International Airlines Now assigned to Boliviana de Aviacion (BoA) | OASIS | Spain | Ceased operations |
| OK | CSA | Czech Airlines | CSA-LINES | Czech Republic |
|
1. ^FAA Notice 7340.343{{dead link|date=December 2017 |bot=InternetArchiveBot |fix-attempted=yes }}
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