| 1 | O&G | Oakdale & Gulf Ry. Co. | 7/1923 | | NAKINA |
| 2 | O&G | OAKDALE & GULF RWY. CO. | | | PWRC |
| 3 | O&W | New York, Ontario & Western Ry. Co. | 8/1920 & 7/1930-7/1936 | | NAKINA |
| 4 | O&W | NEW YORK, ONTARIO & WESTERN RWY. CO. | | | PWRC |
| 5 | O2 | OLT Express Sp z.o.o | ICAO: JEA, Callsign: Jeta, Ceased Operations in July 2012. Name changed from Jet Air Spolka z.o.o. in February 2012. | PL | AVCODES |
| 6 | O2 | Oceanic Airlines Inc. | ICAO: GIO, Designator O2 recalled by IATA. Name changed from Guinee Paramount Airlines S.A. | GUI | AVCODES |
| 7 | O2 | Jet Air Spolka z.o.o. | ICAO: JEA, Callsign: Jeta, Name changed to OLT Express Sp z.o.o | PL | AVCODES |
| 8 | O2 | Hopscotch Air, Inc. | ICAO: HPK, Callsign: Hopscotch Air, | USA | AVCODES |
| 9 | O3 | Bellview Airlines S/L Ltd. | ICAO: BVU, | WAL | AVCODES |
| 10 | O3 | Oriental Air Bridge Co. Ltd. | ICAO: NGK, Callsign: Oriental Bridge, IATA designator changed to OC. ICAO Designator changed to ORC. | J | AVCODES |
| 11 | O3 | Shun Fung Airlines Company Limited | ICAO: CSS, Callsign: Shun Feng, IATA Prefix: 921, | PRC | AVCODES |
| 12 | O3 | Orange Air, LLC | ICAO: ORN, Callsign: Orange Jet, | USA | AVCODES |
| 13 | O4 | Octavia Airlines | ICAO: OCN, Callsign: Octaflight, Ceased operations. | F | AVCODES |
| 14 | O4 | Antrak Air Limited | ICAO: ABV, Callsign: Antrak, | GH | AVCODES |
| 15 | O4 | orange2fly Airlines S.A. | ICAO: OTF, IATA Prefix: 919, Filed for bankruptcy protection on 20 October 2020. Founded in September 2015. Commenced operations on 20 July 2016. Ceased operations on 01 July 2020. | GR | AVCODES |
| 16 | O5 | Orange Air Sierra Leone Limited | ICAO: ORJ, | WAL | AVCODES |
| 17 | O5 | Comores Aviation International | ICAO: KMZ, | COM | AVCODES |
| 18 | O6 | Oceanair Linhas Aereas Ltda | ICAO: ONE, Callsign: Oceanair, IATA Prefix: 247, Name changed to Oceanair Linhas Aereas Ltda t/a Avianca Brazil w.e.f. 26 March 2010. | BR | AVCODES |
| 19 | O6 | Oceanair Linhas Aereas Ltda t/a Avianca Brazil | ICAO: ONE, Callsign: Oceanair, IATA Prefix: 247, Ceased operations on 24 May 2019. Name changed from Oceanair Linhas Aereas Ltda w.e.f. 26 March 2010. | BR | AVCODES |
| 20 | O7 | OzJet Airlines Pty Limited | ICAO: OZJ, Callsign: Ausjet, | AUS | AVCODES |
| 21 | O7 | Eznis Airways | ICAO: EZA, Callsign: Eznis, IATA Prefix: 646, IATA designator changed to EF. IATA designator changed from O7 | MGL | AVCODES |
| 22 | O7 | Omni-blu Aviation Limited | ICAO: OMB, Callsign: Omniblu, | WAN | AVCODES |
| 23 | O8 | Oasis Hong Kong Airlines Limited | ICAO: OHK, Callsign: Oasis, IATA Prefix: 558, Ceased Operations | HK | AVCODES |
| 24 | O8 | Axis International Lines S.A. | | CAM | AVCODES |
| 25 | O8 | Siam Air Transport Co., Ltd. | ICAO: SQM, Callsign: Golden Fin, IATA Prefix: 874, | T | AVCODES |
| 26 | O8 | SNCF-C6 SASU | Bus Service | F | AVCODES |
| 27 | O8 | Marathon Airlines | ICAO: MTO, Callsign: Marathon, | GR | AVCODES |
| 28 | O9 | Indus Airways Private Limited | | IND | AVCODES |
| 29 | O9 | Nova Airlines | ICAO: NOV, Callsign: Novanile, IATA Prefix: 948, | SUD | AVCODES |
| 30 | OA | Olympic Airways | ICAO: OAL, Callsign: Olympic, IATA Prefix: 050, Name changed to Olympic Airlines | GR | AVCODES |
| 31 | OA | Olympic Airlines | ICAO: OAL, Callsign: Olympic, IATA Prefix: 050, IATA designator changed to OP. Name changed from Olympic Airways | GR | AVCODES |
| 32 | OA | Pantheon Airways S.A. d/b/a Olympic Air | ICAO: NOA, Callsign: Pantheon, IATA Prefix: 050, | GR | AVCODES |
| 33 | OA | Olympic Air | ICAO: OAL, Callsign: Olympic, IATA Prefix: 050, | GR | AVCODES |
| 34 | OAA | Oxley Aviation | | AUS | AVCODES |
| 35 | OAA | Impulse Airlines Pty Limited | IATA: VQ, ICAO designator changed to IPU | AUS | AVCODES |
| 36 | OAA | Oxley Aviation | | | ICAO1 |
| 37 | OAAS | OURADA LOGISTICS LLC | | | SCAC4 |
| 38 | OAB | Owners Abroad Aviation Ltd. | | GB | AVCODES |
| 39 | OAB | Osh | Callsign: Osh Air, Ceased operations | KS | AVCODES |
| 40 | OABC | O & B TRANSPORTATION INC | | | SCAC3 |
| 41 | OABI | OWENS & BRAY INC | | | SCAC3 |
| 42 | OABQ | ONE ABC ENTERPRISES (1 ABC ENTERPRI | | | SCAC3 |
| 43 | OAC | Oriental Airlines Limited | Callsign: Oriental Air, | WAN | AVCODES |
| 44 | OAC | Oriental Airlines | | | ICAO1 |
| 45 | OACH | CENTRAL TRANSPORTATION CO. {O.A. COOPER CO.} | | | PWRC |
| 46 | OACH | OUACHITA TERMINALS INC | | | SCAC4 |
| 47 | OACN | OA CARGO & TRANSPORTATION | | | SCAC4 |
| 48 | OACU | LINDE-HADJIKYRIAKOS GAS LTD. | | CY | BIC |
| 49 | OACU | HADJIKYRIAKOS INDUSTRIES LTD | | | SCAC4 |
| 50 | OACX | Central Transportation Co., Inc. | added 7/1983; eliminated 10/1988 | | NAKINA |
| 51 | OACX | The O.A. Cooper Co. | added 1/1974; 1/1975-1/1980; eliminated 5/1983 | | NAKINA |
| 52 | OACX | The O.A. Cooper Co. | added 1/74; 1/75-1/80; eliminated 5/83 | | NRHS |
| 53 | OACX | Central Transportation Co., Inc. | eliminated 10/88 | | NRHS |
| 54 | OACX | CENTRAL TRANSPORTATION CO. | | | PWRC |
| 55 | OACX | Central Transportation | | | RSRV |
| 56 | OAD | Orscom Tourist Installations Company | Callsign: Orscom, | ET | AVCODES |
| 57 | OAD | Orscom Tourist Installations Company | | | ICAO1 |
| 58 | OAD | | | NL | TRAFFIC |
| 59 | OADG | O & A DEHU GREENHOUSES LTD | | | SCAC4 |
| 60 | OADI | OHIO AUTO DELIVERY INC | | | SCAC4 |
| 61 | OAE | Omni Air International, Inc. d/b/a Omni Air Express | IATA: OY, Callsign: Omni-Express, IATA Designator changed from 1I | USA | AVCODES |
| 62 | OAE | Omni Air International, Inc. d/b/a Omni Air Express | IATA: 1I, Callsign: Omni-Express, IATA Designator changed to OY* | USA | AVCODES |
| 63 | OAE | Omni Air International | IATA: OY | | ICAO1 |
| 64 | OAEA | ORANGE AVENUE EXPRESS INC | | | SCAC4 |
| 65 | OAEN | A-ONE (A-1) TRANSPORTATION | | | SCAC4 |
| 66 | OAF | Austrian Air Ambulance | Callsign: Austrain Ambulance, | A | AVCODES |
| 67 | OAFG | O/A FINDLAY GRAIN AND LIVESTOCK HAU | | | SCAC4 |
| 68 | OAFH | OCEANAIR FORWARDING INC | | | SCAC4 |
| 69 | OAFL | O&A TEX-PACK EXPRESS, INC | | | SCAC3 |
| 70 | OAFU | ORCA AFRICA PTY LTD | | ZA | BIC |
| 71 | OAFW | OASIS LOGISTICS LLC | | | SCAC4 |
| 72 | OAG | OLD AUGUSTA RAILROAD COMPANY | | | PWRC |
| 73 | OAHQ | OAHU TRANSPORT CO LTD | | | SCAC3 |
| 74 | OAI | Tor Air AB | Callsign: Torline, AOC granted on 01 December 2008. | S | AVCODES |
| 75 | OAIC | ORVILLES APPLIANCE INC | | | SCAC4 |
| 76 | OAIN | ODOI ASSOCIATES INC | | | SCAC4 |
| 77 | OAIT | O & I TRANSPORT INC | | | SCAC3 |
| 78 | OAIT | O & I TRANSPORT INC | | | SCAC4 |
| 79 | OAK | Oakway | | USA | TRAFFIC |
| 80 | OAKC | OAKMONT CARRIER CORP | | | SCAC3 |
| 81 | OAKD | OAKLAND TRANSPORTATION SERVICE INC | | | SCAC3 |
| 82 | OAKD | OAKLAND TRANSPORTATION INC | | | SCAC4 |
| 83 | OAKE | OAKES, CHARLES H | | | SCAC3 |
| 84 | OAKF | OAKCREST FUEL INC | | | SCAC4 |
| 85 | OAKG | OAK GROVE TRUCKING COMPANY | | | SCAC3 |
| 86 | OAKH | OAK HARBOR FREIGHT LINES, INC | | | SCAC3 |
| 87 | OAKH | OAK HARBOR FREIGHT LINES INC | | | SCAC4 |
| 88 | OAKI | OAKLAND VAN & STORAGE INC | | | SCAC3 |
| 89 | OAKK | OAKLEY TANK LINES INC | | | SCAC4 |
| 90 | OAKL | OAKS LUMBER&HARDWARE, INC | | | SCAC3 |
| 91 | OAKL | OAKLEY DIE & MOLD CO INC | | | SCAC4 |
| 92 | OAKP | OAKLAND PORT SERVICES CORP | | | SCAC4 |
| 93 | OAKQ | OAK PARK TRANSFER | | | SCAC3 |
| 94 | OAKT | OAKS TRAILER TOWING | | | SCAC3 |
| 95 | OAKT | Oakville Transit | | CDN | TRAFFIC |
| 96 | OAKU | OAK RIDGE NATIONAL LABORATORY | | USA | BIC |
| 97 | OAKU | OAKSHOTT LIMITED | | | SCAC3 |
| 98 | OAKV | OAKS VAN LINES | | | SCAC3 |
| 99 | OAKV | OAKHAVEN RANCH INC | | | SCAC4 |
| 100 | OAKW | OAKWOOD TRUCKING CORP | | | SCAC3 |
