| Ain Beida Airport | OGX | Ouargla | DZ |
| Albert J Ellis Airport | OAJ | Richlands | US |
| Anacortes Airport | OTS | Anacortes | US |
| Antoine de Saint Exupéry Airport | OES | San Antonio Oeste | AR |
| Åre Östersund Airport | OSD | Östersund | SE |
| Asturias Airport | OVD | Ranón | ES |
| Balimo Airport | OPU | Balimo | PG |
| Bodaybo Airport | ODO | Bodaybo | RU |
| Bucharest Henri Coandă International Airport | OTP | Otopeni | RO |
| Cacoal Airport | OAL | Cacoal | BR |
| Chicago O'Hare International Airport | ORD | Chicago | US |
| Cooma Snowy Mountains Airport | OOM | Cooma | AU |
| Córdoba Airport | ODB | Córdoba | ES |
| Corisco International Airport | OCS | Corisco Island | GQ |
| Cork International Airport | ORK | Cork | IE |
| Enrique Adolfo Jimenez Airport | ONX | Colón | PA |
| Eppley Airfield | OMA | Omaha | US |
| Eugene F. Correia International Airport | OGL | Ogle | GY |
| Francisco De Orellana Airport | OCC | Coca | EC |
| Francisco de Sá Carneiro Airport | OPO | Porto | PT |
| Gold Coast Airport | OOL | Gold Coast | AU |
| H.E Alfredo Martinez (Tower Hill) Airstrip | ORZ | Orange Walk | BZ |
| Ian Fleming International Airport | OCJ | Boscobel | JM |
| Juan Mendoza International Airport | ORU | Oruro | BO |
| Kahului International Airport | OGG | Kahului | US |
| Kobuk Airport | OBU | Kobuk | US |
| L M Clayton Airport | OLF | Wolf Point | US |
| Labo Airport | OZC | Ozamiz | PH |
| Leoš Janáček Airport Ostrava | OSR | Mošnov | CZ |
| Long Seridan Airport | ODN | Long Seridan | MY |
| Miami-Opa Locka Executive Airport | OPF | Miami | US |
| Mohe Gulian Airport | OHE | Mohe | CN |
| Mornington Island Airport | ONG | — | AU |
| Mostar International Airport | OMO | Mostar | BA |
| Naha International Airport | OKA | Naha | JP |
| Namsos Airport | OSY | Namsos | NO |
| Nome Airport | OME | Nome | US |
| Norfolk International Airport | ORF | Norfolk | US |
| North West Santo Airport | OLJ | Olpoi | VU |
| Northway Airport | ORT | Northway | US |
| Novosibirsk Tolmachevo Airport | OVB | Novosibirsk | RU |
| Oakey Army Aviation Centre | OKY | — | AU |
| Oakland San Francisco Bay Airport | OAK | Oakland | US |
| Oban Airport | OBN | North Connel | GB |
| Obo Airport | OBX | Obo | PG |
| Ocean City Municipal Airport | OCE | Ocean City | US |
| Odate Noshiro Airport | ONJ | Kitaakita | JP |
| Odense Hans Christian Andersen Airport | ODE | Odense | DK |
| Oecusse Route of the Sandalwood International Airport | OEC | Oecussi-Ambeno | TL |
| Ofu Airport | OFU | Ofu | AS |
| Ogden Hinckley Airport | OGD | Ogden | US |
| Ogdensburg International Airport | OGS | Ogdensburg | US |
| Ohrid St. Paul the Apostle Airport | OHD | Ohrid | MK |
| Oita Airport | OIT | Oita | JP |
| Okayama Momotaro Airport | OKJ | Okayama | JP |
| OKC Will Rogers World Airport | OKC | Oklahoma City | US |
| Okhotsk Airport | OHO | Okhotsk | RU |
| Oki Global Geopark Airport | OKI | Okinoshima | JP |
| Okinoerabu Airport | OKE | Wadomari | JP |
| Oksibil Airport | OKL | Oksibil | ID |
| Okushiri Airport | OIR | Okushiri Island | JP |
| Olbia Costa Smeralda Airport | OLB | Olbia (SS) | IT |
| Old Harbor Airport | OLH | Old Harbor | US |
| Olympia Regional Airport | OLM | Olympia | US |
| Olympic Dam Airport | OLP | Olympic Dam | AU |
| Olyokminsk Airport | OLZ | Olyokminsk | RU |
| Omsk Central Airport | OMS | Omsk | RU |
| Ondangwa Airport | OND | Ondangwa | NA |
| Ontario International Airport | ONT | Ontario | US |
| Oradea International Airport | OMR | Oradea | RO |
| Oran Es-Sénia (Ahmed Ben Bella) International Airport | ORN | Es-Sénia | DZ |
| Oranjemund Airport | OMD | Oranjemund | NA |
| Ordu–Giresun Airport | OGU | Ordu | TR |
| Örebro Airport | ORB | Örebro | SE |
| Ørland Airport | OLA | Ørland | NO |
| Örnsköldsvik Airport | OER | Örnsköldsvik | SE |
| Orsk Airport | OSW | Orsk | RU |
| Osh International Airport | OSS | Osh | KG |
| Oshima Airport | OIM | Izu Oshima | JP |
| Osijek Airport | OSI | Osijek(Klisa) | HR |
| Oslo-Gardermoen International Airport | OSL | Oslo (Gardermoen) | NO |
| Ostend-Bruges International Airport | OST | Oostende | BE |
| Osvaldo Vieira International Airport | OXB | Bissau | GW |
| Ouagadougou Thomas Sankara International Airport | OUA | Ouagadougou | BF |
| Ouarzazate International Airport | OZZ | Ouarzazate | MA |
| Oudomsay Airport | ODY | Oudomsay | LA |
| Ouessant Airport | OUI | Ushant | FR |
| Oujda Angads Airport | OUD | Ahl Angad | MA |
| Oulu Airport | OUL | Oulu / Oulunsalo | FI |
| Owensboro Daviess County Airport | OWB | Owensboro | US |
| Oyem Airport | OYE | Oyem | GA |
| Paris-Orly Airport | ORY | Paris (Orly, Val-de-Marne) | FR |
| Port Lions Airport | ORI | Port Lions | US |
| Presidente João Batista Figueiredo Airport | OPS | Sinop | BR |
| Ralph Wien Memorial Airport | OTZ | Kotzebue | US |
| Raul Arias Espinoza Airport | OTD | Contadora Island | PA |
| Robert (Bob) Curtis Memorial Airport | ORV | Noorvik | US |
| Salinópolis Airport | OPP | Salinópolis | BR |
| Sapporo Okadama Airport | OKD | Sapporo | JP |
| Southwest Oregon Regional Airport | OTH | North Bend | US |
| Sovetskiy Airport | OVS | Sovetskiy | RU |
| Suhar International Airport | OHS | Suhar | OM |
| Tanjung Api Airport | OJU | Tojo Una-Una | ID |
| Tazadit Airport | OUZ | Zouérate | MR |
| Tokachi-Obihiro Airport | OBO | Obihiro | JP |
| Toksook Bay Airport | OOK | Toksook Bay | US |
| Urmia Airport | OMH | Urmia | IR |
| Vladikavkaz Beslan International Airport | OGZ | Beslan | RU |
| Worcester Regional Airport | ORH | Worcester | US |
| Xoxocotlán International Airport | OAX | Oaxaca | MX |
| Yonaguni Airport | OGN | Yonaguni | JP |
| Yorke Island Airport | OKR | Yorke Island | AU |
| Zagora Airport | OZG | Zagora | MA |
| Zomin Airport | OMN | Zomin | UZ |
| Zonguldak Çaycuma Airport | ONQ | Zonguldak | TR |
| Zorg en Hoop Airport | ORG | Paramaribo | SR |