| 3901 | OTTX | Trailer Train Co. | 7/65; added 10/65; 10/66 | | NRHS |
| 3902 | OTTX | Trailer Train Co. (FMS flats specially equipped to provide for loading of agricultural equipment) | Oct-68 | | NRHS |
| 3903 | OTTX | Trailer Train Co. | Apr-70 | | NRHS |
| 3904 | OTTX | Trailer Train Co. (flat cars equipped with 64 chains, with snubbers, each secured to movable and retractable tie-down winches in four longitudinal channels, for transporting agricultural equipment) | 1/75-1/80 | | NRHS |
| 3905 | OTTX | Trailer Train Co. | eliminated 1/92 | | NRHS |
| 3906 | OTTX | TTX Co. | added 1/92 | | NRHS |
| 3907 | OTTX | TTX Co. (flat cars equipped with 48 chains, with snubbers, each secured to movable and retractable tie-down winches in 4 longitudinal channels; for transporting agricultural equipment) | Jan-94 | | NRHS |
| 3908 | OTTX | TTX Co. (flat cars equipped with 48 chains, with snubbers, each secured to movable and retractable tie-down winches in 4 longitudinal channels; for transporting agricultural equipment, equipped with end of car cush.) | Jul-00 | | NRHS |
| 3909 | OTTX | TTX COMPANY | | | PWRC |
| 3910 | OTTX | Trailer Train (TTX Corporation) | | | RSRV |
| 3911 | OTTX | TTX COMPANY | | | SCAC4 |
| 3912 | OTUA | OXXO TRANSPORT USA | | | SCAC4 |
| 3913 | OTUM | OTTUMWA CONNECTING RAILROAD | | | PWRC |
| 3914 | OTUS | ODFJELL TANKERS (USA) INC. | | | SCAC3 |
| 3915 | OTUS | ODFJELL SEACHEM AS | | | SCAC4 |
| 3916 | OTVA | ONE TWO SEVEN ZERO ZERO FOUR NINE ( | | | SCAC4 |
| 3917 | OTVB | ORLEANS TRAVEL BUREAU INC | | | SCAC3 |
| 3918 | OTVC | ON TIME SERVICES INC | | | SCAC3 |
| 3919 | OTVD | OLYMPIC TRANSPORTATION SERVICES | | | SCAC4 |
| 3920 | OTVE | ON TIME SERVICIOS CONSOLIDADOS SA D | | | SCAC4 |
| 3921 | OTVR | Otter Tail Valley R.R. Co., Inc. | added 4/1987; 1/1994; eliminated 10/1999; 7/2000-7/2005 | | NAKINA |
| 3922 | OTVR | Otter Tail Valley R.R. Co., Inc. | added 4/87; 1/94; eliminated 10/99; 7/2000 | | NRHS |
| 3923 | OTVR | OTTER TAIL VALLEY RAILROAD CO., INC. (RAIL AMERICA) | | | PWRC |
| 3924 | OTVR | Otter Tail Valley Railroad (Genesee & Wyoming) | | | RSRV |
| 3925 | OTVR | OTTERTAIL VALLEY RAILROAD CO INC | | | SCAC3 |
| 3926 | OTVS | ONE THREE SEVEN SEVEN ZERO SIX SEVE | | | SCAC4 |
| 3927 | OTWA | OTTAWA TRANSPORT, INC | | | SCAC3 |
| 3928 | OTWC | ONE TWO NINE TWO SEVEN THREE (12927 | | | SCAC4 |
| 3929 | OTWD | ON THE WATER DESIGNS | | | SCAC4 |
| 3930 | OTWE | ONE TWO TWO ONE NINE NINE THREE (12 | | | SCAC4 |
| 3931 | OTWI | O T | | | SCAC4 |
| 3932 | OTWK | OUTLAW TRUCKING LLC | | | SCAC4 |
| 3933 | OTWO | OTTAWA OIL COMPANY | | | SCAC4 |
| 3934 | OTWQ | ON THE WAY TRANSPORT INTERNATIONAL | | | SCAC4 |
| 3935 | OTWS | ONE THREE SEVEN TWO (1372) TANKER C | | | SCAC4 |
| 3936 | OTWV | ONE TWO SEVEN SEVEN ONE ONE NINE ( | | | SCAC4 |
| 3937 | OTWW | ON-TIME WORLDWIDE | | | SCAC4 |
| 3938 | OTWX | OF THE WOODS, LLC. | | | PWRC |
| 3939 | OTWX | OF THE WOODS LLC | | | SCAC4 |
| 3940 | OTX | Aerotex | Callsign: Zotex, | KZ | AVCODES |
| 3941 | OTX | Oliver Tank Line | 7/1923 | | NAKINA |
| 3942 | OTX | OLIVER TANK LINE | | | PWRC |
| 3943 | OTXC | ON TIME TRANSPORT LLC | | | SCAC4 |
| 3944 | OTXI | OLYMPIC TRANSPORT INC | | | SCAC3 |
| 3945 | OTXK | ON TIME TRANSPORTATION | | | SCAC4 |
| 3946 | OTXL | ON TIME LOGISTICS LLC | | | SCAC4 |
| 3947 | OTXN | ORAMS TRANSPORTATION | | | SCAC4 |
| 3948 | OTXP | OLD TIME EXPRESS INC | | | SCAC4 |
| 3949 | OTXQ | OTI TRANSPORT INC | | | SCAC4 |
| 3950 | OTXR | ON TIME EXPRESS INC | | | SCAC4 |
| 3951 | OTXT | OSOS TRANSPORT | | | SCAC4 |
| 3952 | OTXV | OUR TRUCK TRANSPORT SERVICES | | | SCAC4 |
| 3953 | OTYI | O T Y INC | | | SCAC3 |
| 3954 | OTYW | ORIENT TRIBUNE SHIPPING COMPANY LIM | | | SCAC4 |
| 3955 | OTZ | Ötztaler Verkehrsgesellschaft mbH | | A | TRAFFIC |
| 3956 | OTZB | ONE TWO TWO ZERO SIX SEVEN EIGHT (1 | | | SCAC4 |
| 3957 | OTZE | ONE THREE ZERO ZERO (1300) CAPITAL | | | SCAC4 |
| 3958 | OTZI | ORTIZ BROTHERS TRUCKING INC | | | SCAC3 |
| 3959 | OTZI | ORTIZ TRUCKING | | | SCAC4 |
| 3960 | OTZL | ONE THREE ZERO SEVEN SEVEN FIVE | | | SCAC4 |
| 3961 | OTZM | ONE THREE ZERO NINE SIX SIX SIX (13 | | | SCAC4 |
| 3962 | OTZS | ONE THREE ZERO SEVEN SIX ONE NINE ( | | | SCAC4 |
| 3963 | OTZT | ONE FIVE THREE ZERO THREE FOUR (153 | | | SCAC4 |
| 3964 | OU | Croatia Airlines | ICAO: CTN, Callsign: Croatia, IATA Prefix: 831, | HR | AVCODES |
| 3965 | OU | Air Atonabee Limited d/b/a City Express | ICAO: OUL, Callsign: City Express, Designator OU recalled by IATA | CDN | AVCODES |
| 3966 | OU | Court Line Aviation Ltd. | Callsign: Court Line, Ceased operations | GB | AVCODES |
| 3967 | OU | Sultan of Oman Air Force | ICAO: OMN, Callsign: Oscar Uniform, ICAO callsign changed to Majan | OM | AVCODES |
| 3968 | OU | Sultan of Oman Air Force | ICAO: OMN, Callsign: Majan, Designator OU recalled by IATA. ICAO callsign changed from Oscar Uniform | OM | AVCODES |
| 3969 | OUA | Transport Aerien Ouest African | Callsign: Quest African, | SN | AVCODES |
| 3970 | OUAK | OUTDOOR ALASKA | | | SCAC3 |
| 3971 | OUAK | OUR OIL LLC | | | SCAC4 |
| 3972 | OUAO | OUGHTON, A & SONS INC | | | SCAC3 |
| 3973 | OUBE | OUBRE, IVORY A TRUCKING INC | | | SCAC4 |
| 3974 | OUCH | Ouachita R.R. | added 4/1991; 1/1994-7/2005 | | NAKINA |
| 3975 | OUCH | Quachita R.R. | added 4/91; 1/94-7/2000 | | NRHS |
| 3976 | OUCH | OUACHITA RAILROAD | | | PWRC |
| 3977 | OUCH | Ouachita Railroad | | | RSRV |
| 3978 | OUCH | OUACHITA RAILROAD | | | SCAC3 |
| 3979 | OUCH | OUACHITA RAILROAD | | | SCAC4 |
| 3980 | OUCO | ORLANDO UTILITIES COMMISSION | | | SCAC4 |
| 3981 | OUCX | Orlando Utilities Commission | added 1/1987; 1/1994-7/2005 | | NAKINA |
| 3982 | OUCX | Orlando Utilities Commission | added 1/87; 1/94-7/2000 | | NRHS |
| 3983 | OUCX | ORLANDO UTILITIES COMMISSION | | | PWRC |
| 3984 | OUCX | Orlando Utilities Commission | | | RSRV |
| 3985 | OUCX | ORLANDO UTILITIES COMMISSION | | | SCAC4 |
| 3986 | OUCZ | OZARK UTILITIES COMPANY | | | PWRC |
| 3987 | OUEL | OUELLET, RAYNOLD TRANSPORT INC | | | SCAC3 |
| 3988 | OUEL | OUELLET, RAYNALD | | | SCAC4 |
| 3989 | OUEQ | OUELLETTE, LUCIEN, CARTAGE | | | SCAC3 |
| 3990 | OUEQ | OUELLETTE, LUCIEN CARTAGE | | | SCAC4 |
| 3991 | OUEST | Chemins de fer de l'Ouest | | F | TRAFFIC |
| 3992 | OUF | Beijing Eofa International Jet Co., Ltd. | Callsign: Element, | PRC | AVCODES |
| 3993 | OUIE | OUI EXPRESS | | | SCAC3 |
| 3994 | OUIES | Ouigo España | | E | TRAFFIC |
| 3995 | OUIJ | OUIMET TRANSPORT | | | SCAC4 |
| 3996 | OUIM | OUIMETTE, ALAN JOHN | | | SCAC4 |
| 3997 | OUL | Oklahoma Aviation LLC d/b/a Soulbird | Callsign: Soul Bird, | USA | AVCODES |
| 3998 | OUL | Air Atonabee Limited d/b/a City Express | IATA: OU, Callsign: City Express, Designator OU recalled by IATA | CDN | AVCODES |
| 3999 | OUL | Air Atonabee | | | ICAO1 |
| 4000 | OULF | OUELLETTE FARMS | | | SCAC3 |
