| 1 | OA | Olympic Airways | ICAO: OAL, Callsign: Olympic, IATA Prefix: 050, Name changed to Olympic Airlines | GR | AVCODES |
| 2 | OA | Olympic Airlines | ICAO: OAL, Callsign: Olympic, IATA Prefix: 050, IATA designator changed to OP. Name changed from Olympic Airways | GR | AVCODES |
| 3 | OA | Pantheon Airways S.A. d/b/a Olympic Air | ICAO: NOA, Callsign: Pantheon, IATA Prefix: 050, | GR | AVCODES |
| 4 | OA | Olympic Air | ICAO: OAL, Callsign: Olympic, IATA Prefix: 050, | GR | AVCODES |
| 5 | OAA | Oxley Aviation | | AUS | AVCODES |
| 6 | OAA | Impulse Airlines Pty Limited | IATA: VQ, ICAO designator changed to IPU | AUS | AVCODES |
| 7 | OAA | Oxley Aviation | | | ICAO1 |
| 8 | OAAS | OURADA LOGISTICS LLC | | | SCAC4 |
| 9 | OAB | Owners Abroad Aviation Ltd. | | GB | AVCODES |
| 10 | OAB | Osh | Callsign: Osh Air, Ceased operations | KS | AVCODES |
| 11 | OABC | O & B TRANSPORTATION INC | | | SCAC3 |
| 12 | OABI | OWENS & BRAY INC | | | SCAC3 |
| 13 | OABQ | ONE ABC ENTERPRISES (1 ABC ENTERPRI | | | SCAC3 |
| 14 | OAC | Oriental Airlines Limited | Callsign: Oriental Air, | WAN | AVCODES |
| 15 | OAC | Oriental Airlines | | | ICAO1 |
| 16 | OACH | CENTRAL TRANSPORTATION CO. {O.A. COOPER CO.} | | | PWRC |
| 17 | OACH | OUACHITA TERMINALS INC | | | SCAC4 |
| 18 | OACN | OA CARGO & TRANSPORTATION | | | SCAC4 |
| 19 | OACU | LINDE-HADJIKYRIAKOS GAS LTD. | | CY | BIC |
| 20 | OACU | HADJIKYRIAKOS INDUSTRIES LTD | | | SCAC4 |
| 21 | OACX | Central Transportation Co., Inc. | added 7/1983; eliminated 10/1988 | | NAKINA |
| 22 | OACX | The O.A. Cooper Co. | added 1/1974; 1/1975-1/1980; eliminated 5/1983 | | NAKINA |
| 23 | OACX | The O.A. Cooper Co. | added 1/74; 1/75-1/80; eliminated 5/83 | | NRHS |
| 24 | OACX | Central Transportation Co., Inc. | eliminated 10/88 | | NRHS |
| 25 | OACX | CENTRAL TRANSPORTATION CO. | | | PWRC |
| 26 | OACX | Central Transportation | | | RSRV |
| 27 | OAD | Orscom Tourist Installations Company | Callsign: Orscom, | ET | AVCODES |
| 28 | OAD | Orscom Tourist Installations Company | | | ICAO1 |
| 29 | OAD | | | NL | TRAFFIC |
| 30 | OADG | O & A DEHU GREENHOUSES LTD | | | SCAC4 |
| 31 | OADI | OHIO AUTO DELIVERY INC | | | SCAC4 |
| 32 | OADK | Okresní automobilová doprava Kolín, s.r.o. | | CZ | TRAFFIC |
| 33 | OAE | Omni Air International, Inc. d/b/a Omni Air Express | IATA: OY, Callsign: Omni-Express, IATA Designator changed from 1I | USA | AVCODES |
| 34 | OAE | Omni Air International, Inc. d/b/a Omni Air Express | IATA: 1I, Callsign: Omni-Express, IATA Designator changed to OY* | USA | AVCODES |
| 35 | OAE | Omni Air International | IATA: OY | | ICAO1 |
| 36 | OAEA | ORANGE AVENUE EXPRESS INC | | | SCAC4 |
| 37 | OAEN | A-ONE (A-1) TRANSPORTATION | | | SCAC4 |
| 38 | OAF | Austrian Air Ambulance | Callsign: Austrain Ambulance, | A | AVCODES |
| 39 | OAF | Österreichische Automobil-Fabrik | Original name: Österreichische Austro-Fiat | A | TRAFFIC |
| 40 | OAFG | O/A FINDLAY GRAIN AND LIVESTOCK HAU | | | SCAC4 |
| 41 | OAFH | OCEANAIR FORWARDING INC | | | SCAC4 |
| 42 | OAFL | O&A TEX-PACK EXPRESS, INC | | | SCAC3 |
| 43 | OAFU | ORCA AFRICA PTY LTD | | ZA | BIC |
| 44 | OAFW | OASIS LOGISTICS LLC | | | SCAC4 |
| 45 | OAG | OLD AUGUSTA RAILROAD COMPANY | | | PWRC |
| 46 | OAHQ | OAHU TRANSPORT CO LTD | | | SCAC3 |
| 47 | OAI | Tor Air AB | Callsign: Torline, AOC granted on 01 December 2008. | S | AVCODES |
| 48 | OAIC | ORVILLES APPLIANCE INC | | | SCAC4 |
| 49 | OAIN | ODOI ASSOCIATES INC | | | SCAC4 |
| 50 | OAIT | O & I TRANSPORT INC | | | SCAC3 |
| 51 | OAIT | O & I TRANSPORT INC | | | SCAC4 |
| 52 | OAK | Oakway | | USA | TRAFFIC |
| 53 | OAKC | OAKMONT CARRIER CORP | | | SCAC3 |
| 54 | OAKD | OAKLAND TRANSPORTATION SERVICE INC | | | SCAC3 |
| 55 | OAKD | OAKLAND TRANSPORTATION INC | | | SCAC4 |
| 56 | OAKE | OAKES, CHARLES H | | | SCAC3 |
| 57 | OAKF | OAKCREST FUEL INC | | | SCAC4 |
| 58 | OAKG | OAK GROVE TRUCKING COMPANY | | | SCAC3 |
| 59 | OAKH | OAK HARBOR FREIGHT LINES, INC | | | SCAC3 |
| 60 | OAKH | OAK HARBOR FREIGHT LINES INC | | | SCAC4 |
| 61 | OAKI | OAKLAND VAN & STORAGE INC | | | SCAC3 |
| 62 | OAKK | OAKLEY TANK LINES INC | | | SCAC4 |
| 63 | OAKL | OAKS LUMBER&HARDWARE, INC | | | SCAC3 |
| 64 | OAKL | OAKLEY DIE & MOLD CO INC | | | SCAC4 |
| 65 | OAKP | OAKLAND PORT SERVICES CORP | | | SCAC4 |
| 66 | OAKQ | OAK PARK TRANSFER | | | SCAC3 |
| 67 | OAKT | OAKS TRAILER TOWING | | | SCAC3 |
| 68 | OAKT | Oakville Transit | | CDN | TRAFFIC |
| 69 | OAKU | OAK RIDGE NATIONAL LABORATORY | | USA | BIC |
| 70 | OAKU | OAKSHOTT LIMITED | | | SCAC3 |
| 71 | OAKV | OAKS VAN LINES | | | SCAC3 |
| 72 | OAKV | OAKHAVEN RANCH INC | | | SCAC4 |
| 73 | OAKW | OAKWOOD TRUCKING CORP | | | SCAC3 |
| 74 | OAKY | OAKRIDGE TRUCKING | | | SCAC4 |
| 75 | OAL | Olympic Airways | IATA: OA, Callsign: Olympic, IATA Prefix: 050, Name changed to Olympic Airlines | GR | AVCODES |
| 76 | OAL | Olympic Airlines | IATA: OA, Callsign: Olympic, IATA Prefix: 050, IATA designator changed to OP. Name changed from Olympic Airways | GR | AVCODES |
| 77 | OAL | Olympic Airlines | IATA: OP, Callsign: Olympic, IATA Prefix: 050, IATA designator changed from OA. Name changed from Olympic Airways | GR | AVCODES |
| 78 | OAL | Olympic Air | IATA: OA, Callsign: Olympic, IATA Prefix: 050, | GR | AVCODES |
| 79 | OAL | Olympic Airlines | IATA: OA | | ICAO1 |
| 80 | OALU | OCEAN AFRICA CONTAINER LINE PTY LTD | | | SCAC4 |
| 81 | OALX | Oregon-American Lumber Corp. | 7/1937-1/1952; eliminated 1/1953 | | NAKINA |
| 82 | OALX | Oregon-American Lumber Corp. | 4/38-1/52; eliminated 1/53 | | NRHS |
| 83 | OALX | OREGON-AMERICAN LUMBER CORP. | | | PWRC |
| 84 | OAMX | Omaha Alfalfa Milling Co. | 8/1920-7/1923 | | NAKINA |
| 85 | OAMX | OMAHA ALFALFA MILLING CO. | | | PWRC |
| 86 | OAN | Aerospace One S.A. | IATA: IW, Callsign: Hellas Cargo, IATA Prefix: 271, | GR | AVCODES |
| 87 | OAN | NATO North Atlantic Treaty Organisation | Callsign: OTAN, | B | AVCODES |
| 88 | OANI | OCEANICA PRODUCTS | | | SCAC4 |
| 89 | OAO | Arkhangelsk Airlines | IATA: 5N, Callsign: Dvina, IATA Prefix: 316, Name changed to Aeroflot - Nord. ICAO codes changed to AUL. IATA Designator changed from 5N* | RUS | AVCODES |
| 90 | OAO | Arkhangelsk Airlines | IATA: 5N, Callsign: Dvina, IATA Designator changed to 5N | RUS | AVCODES |
| 91 | OAO | Arkhangelsk 2 Aviation Division | | | ICAO1 |
| 92 | OAP | Coapa Air, S.A. de C.V. | Callsign: Coapa, | MEX | AVCODES |
| 93 | OAP | COAPA AIR | | | ICAO1 |
| 94 | OAQC | OMEGA-3 AQUACULTURE | | | SCAC4 |
| 95 | OAR | Grupo One Air Aviacion, S.L. | Callsign: Boss Air, | E | AVCODES |
| 96 | OAR | Old Augusta R.R. Co. | added 7/1983; 1/1994-7/2005; eliminated 7/2006 | | NAKINA |
| 97 | OAR | Old Augusta R.R. LLC | added 7/2006 | | NAKINA |
| 98 | OAR | Old Augusta R.R. Co. | 1/94-7/2000 | | NRHS |
| 99 | OAR | OLD AUGUSTA RAILROAD COMPANY | | | PWRC |
| 100 | OAR | Old Augusta Railroad | | | RSRV |
