| 1 | ON | Air Nauru | ICAO: RON, Callsign: Air Nauru, IATA Prefix: 123, Commenceds operation on 14 February 1970. Ceased operations on 18 December 2005. Relaunched as Nauru Air Corporation t/a Our Airline on 14 October 2006. Designator ON reinstated by IATA | NAU | AVCODES |
| 2 | ON | Nauru Air Corporation t/a Our Airline | ICAO: RON, IATA Prefix: 123, Nauru Air Corporation t/a Nauru Airlines w.e.f. 01 August 2014. Name changed from Air Nauru on 14 October 2005. | NAU | AVCODES |
| 3 | ON | Nauru Air Corporation t/a Nauru Airlines | ICAO: RON, IATA Prefix: 123, Name changed from Nauru Air Corporation t/a Our Airline w.e.f. 01 August 2014. | NAU | AVCODES |
| 4 | ON&P | Oklahoma, New Mexico & Pacific Ry. | 8/1920-7/1923 | | NAKINA |
| 5 | ON&P | OKLAHOMA, NEW MEXICO & PACIFIC RWY. | | | PWRC |
| 6 | ONA | Yeongnam Air Co. Ltd. | IATA: OE, Callsign: Yeongnam Air, Founded in 2008. Commenced operations on 25 July 2008. Ceased operations in December 2008. | ROK | AVCODES |
| 7 | ONAI | ONALA TRUCKING CORP | | | SCAC4 |
| 8 | ONAL | O'NEAL, TOLAN TRANSPORTATION & LOGI | | | SCAC3 |
| 9 | ONAN | ONE FOUR FOUR FIVE NINE ONE THREE ( | | | SCAC4 |
| 10 | ONAV | OCEANIC NAVIGATION CORPORATION | | | SCAC3 |
| 11 | ONBD | ONEIL BROTHERS DISTRIBUTING COMPANY | | | SCAC3 |
| 12 | ONBE | ONBEGO COMPANY S A | | | SCAC3 |
| 13 | ONBR | ONE THREE TWO TWO ONE NINE FOUR (13 | | | SCAC4 |
| 14 | ONBT | OWNBY TRUCKING | | | SCAC3 |
| 15 | ONBT | OWNBY TRUCKING | | | SCAC4 |
| 16 | ONBU | OCEANUS CONTAINER LINES PTE LTD | | MAL | BIC |
| 17 | ONC | ONONDAGA COACH CORP | | | SCAC4 |
| 18 | ONCC | OCEAN NAVIGATION 1 CO LTD | | | SCAC4 |
| 19 | ONCD | O'NEILL INCORPORATED | | | SCAC4 |
| 20 | ONCE | ONE CALL DOES IT ALL TRANSPORTATION | | | SCAC3 |
| 21 | ONCE | ONTARIO CONFERENCE OF THE SEVENTH D | | | SCAC4 |
| 22 | ONCF | Office National des Chemin de Fer / Moroccan National Railways | | MA | TRAFFIC |
| 23 | ONCF | Org.Nat. Des C.deF Maroquins | | MA | VKM |
| 24 | ONCM | OREG NEV CALIF FAST FRT | | | PWRC |
| 25 | ONCO | ONEAL CONTRACTING INC | | | SCAC3 |
| 26 | ONCP | ONEAL COMPANY INC | | | SCAC3 |
| 27 | ONCP | ONEIL COMPANY INC | | | SCAC4 |
| 28 | ONCR | ONC CORPORATION | | | SCAC3 |
| 29 | ONCT | Ontario Central R.R. Corp. | added 1/1980; 1/1980-7/2005 | | NAKINA |
| 30 | ONCT | Ontario Central R.R. Corp. | added 1/80; 1/80-7/2000 | | NRHS |
| 31 | ONCT | ONTARIO CENTRAL RAILROAD CORPORATION | | | PWRC |
| 32 | ONCT | Ontario Central Railroad | | | RSRV |
| 33 | ONCT | ONTARIO CENTRAL RAILROAD CORPORATIO | | | SCAC3 |
| 34 | ONCX | OPERATION NATIONAL CONCERN | | | PWRC |
| 35 | ONCX | OPERATION NATIONAL CONCERN | | | SCAC4 |
| 36 | ONCZ | OWENS NEWS CORP. | | | PWRC |
| 37 | OND | Condomett | Callsign: Condomett, | I | AVCODES |
| 38 | OND | PENNYRILE MOVING & | | | SCAC4 |
| 39 | ONDA | ONDA NAVIGATION INC. | | | SCAC3 |
| 40 | ONDA | ONE FIVE NINE NINE SEVEN SIX ONE | | | SCAC4 |
| 41 | ONDB | ONONDAGA BEVERAGE TRANSPORT INC | | | SCAC3 |
| 42 | ONDC | ONTARIO DISPOSAL A DIVISION OF | | | SCAC4 |
| 43 | ONDD | ONTARIO DELIVERY DISPATCH LTD | | | SCAC4 |
| 44 | ONDG | ONONDAGA CORPORATION | | | SCAC3 |
| 45 | ONDO | ONDA OIL COMPANY | | | SCAC4 |
| 46 | ONDS | ONDA SHIPPING COMPANY, LTD. | | | SCAC3 |
| 47 | ONDS | ONE FOUR SIX ONE SEVEN ONE ONE (146 | | | SCAC4 |
| 48 | ONE | Oceanair Linhas Aereas Ltda | IATA: O6, Callsign: Oceanair, IATA Prefix: 247, Name changed to Oceanair Linhas Aereas Ltda t/a Avianca Brazil w.e.f. 26 March 2010. | BR | AVCODES |
| 49 | ONE | Oceanair Linhas Aereas Ltda t/a Avianca Brazil | IATA: O6, 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 |
| 50 | ONE | Oceanair | IATA: O6 | | ICAO1 |
| 51 | ONE | one (London Eastern Railway Ltd,) | ‘one’ is the brand name of London Eastern Railway Ltd, which operates local, suburban and express services from London Liverpool Street in the City of London to East and North London, Essex, Hertfordshire, Cambridgeshire and East Anglia, otherwise known as the Greater Anglia network. ‘one’ is part of the National Express Group. The Greater Anglia network was awarded to a single operator as part of a drive to reduce the number of train companies operating to major termini, in an effort to improve integration between services and reliability. On April 1, 2004 ‘one’ took over all services formerly operated by Anglia Railways and First Great Eastern, as well as the ‘West Anglia’ services of WAGN. The company is now the sole passenger operator on most of the Great Eastern | GB | TRAFFIC |
| 52 | ONE | One (Greater Anglia Rail Franchise) | | UK | VKM |
| 53 | ONEA | ONEAL, CARL TRUCK LINE, INC | | | SCAC3 |
| 54 | ONEB | ORANGE NEWARK ELIZABETH BUS INC | | | SCAC3 |
| 55 | ONEB | ONE ONE SEVEN FIVE ZERO FOUR | | | SCAC4 |
| 56 | ONED | ONEIDA-COLUMBIA EXPRESS COMPANY | | | SCAC3 |
| 57 | ONED | ONE STOP KITCHEN IDEA LTD | | | SCAC4 |
| 58 | ONEE | ONEALES TRANSPORT INC | | | SCAC3 |
| 59 | ONEE | ONEALES TRANSPORT INC | | | SCAC4 |
| 60 | ONEF | ONE FIVE ONE FOUR | | | SCAC4 |
| 61 | ONEG | ONEGO SHIPPING AND CHARTERING B V | | | SCAC4 |
| 62 | ONEH | ONE HORN TRUCKING | | | SCAC4 |
| 63 | ONEI | OVERNIGHT NEW ENGLAND INC | | | SCAC3 |
| 64 | ONEJ | ONEAL, CLAUDE C JR | | | SCAC3 |
| 65 | ONEJ | ONE A (1A) ENTERPRISES | | | SCAC4 |
| 66 | ONEL | ONEL TRANSPORTATION INC | | | SCAC4 |
| 67 | ONEN | ONTARIO-NEW ENGLAND EXPRESS INC | | | SCAC4 |
| 68 | ONEP | ONE (#1) PRIORITY TRANSPORT | | | SCAC3 |
| 69 | ONEQ | O N E SHIPPING INC. | | | SCAC3 |
| 70 | ONEQ | ONE SEVEN EIGHT FOUR FOUR TWO FOUR | | | SCAC4 |
| 71 | ONER | Ontario Eastern R.R. Corp. | added 7/1982; eliminated 4/1986 | | NAKINA |
| 72 | ONER | Ontario Eastern R.R. Corp. | added 7/82; eliminated 4/86 | | NRHS |
| 73 | ONER | ONTARIO EASTERN RAILROAD | | | PWRC |
| 74 | ONER | Ontario Eastern Railroad | | | RSRV |
| 75 | ONER | OHIO NORTHERN EXPRESS INC | | | SCAC4 |
| 76 | ONES | O N E SHIPPING LTD | | | SCAC3 |
| 77 | ONES | ONE FIVE NINE EIGHT SIX SIX FIVE (1 | | | SCAC4 |
| 78 | ONET | ONEAL TRUCKING | | | SCAC3 |
| 79 | ONEU | OCEAN NETWORK EXPRESS PTE. LTD. | | SGP | BIC |
| 80 | ONEU | ONE WAY LEASE INC | | | SCAC4 |
| 81 | ONEW | ONE WORLD LOGISTICS OF AMERICA INC | | | SCAC4 |
| 82 | ONEX | Oneta Refining Co. | 8/1920-7/1923 | | NAKINA |
| 83 | ONEX | ONETA REFINING CO. | | | PWRC |
| 84 | ONEY | ORATEL NETWORKS | | | SCAC4 |
| 85 | ONFA | O'NEIL (FIVE) 5 STAR LLC | | | SCAC4 |
| 86 | ONFJ | Odense - Nørre Broby - Fåborg Jernbane (Brobybanen) | 1906 - 1954 | DK | jernbanen.dk |
| 87 | ONFJ | Odense - Nørre Broby - Fåborg Jernbane (Brobybanen) | 1906 - 1954 | DK | TRAFFIC |
| 88 | ONFS | OLSEN FUEL SUPPLY INC | | | SCAC4 |
| 89 | ONG | Sonnig AG | Callsign: Sonnig, | CH | AVCODES |
| 90 | ONG | Sonnig SA | | | ICAO1 |
| 91 | ONGC | OIL NAVIGATION CORP PIRAEUS GREECE | | | SCAC3 |
| 92 | ONGM | OCEAN AND GULF MARITIME | | | SCAC3 |
| 93 | ONGY | ONGLEY TRUCKING INC | | | SCAC4 |
| 94 | ONHC | ORIENT TIDE SHIPPING COMPANY LIMITE | | | SCAC4 |
| 95 | ONHE | ONE ZERO EIGHT TRANSPORT LTD (108 T | | | SCAC3 |
| 96 | ONHE | OLSON, ROB TRUCKING | | | SCAC4 |
| 97 | ONHL | ONEIDA NEW HOLLAND | | | SCAC4 |
| 98 | ONHR | ONE HOUR DELIVERY (A DIV OF DRYDEN- | | | SCAC3 |
| 99 | ONI | Omni Aviation Training Center | Callsign: Omni Training, | P | AVCODES |
| 100 | ONIE | ONTIME | | | SCAC4 |
