| 1 | OU | Croatia Airlines | ICAO: CTN, Callsign: Croatia, IATA Prefix: 831, | HR | AVCODES |
| 2 | OU | Air Atonabee Limited d/b/a City Express | ICAO: OUL, Callsign: City Express, Designator OU recalled by IATA | CDN | AVCODES |
| 3 | OU | Court Line Aviation Ltd. | Callsign: Court Line, Ceased operations | GB | AVCODES |
| 4 | OU | Sultan of Oman Air Force | ICAO: OMN, Callsign: Oscar Uniform, ICAO callsign changed to Majan | OM | AVCODES |
| 5 | OU | Sultan of Oman Air Force | ICAO: OMN, Callsign: Majan, Designator OU recalled by IATA. ICAO callsign changed from Oscar Uniform | OM | AVCODES |
| 6 | OUA | Transport Aerien Ouest African | Callsign: Quest African, | SN | AVCODES |
| 7 | OUAK | OUTDOOR ALASKA | | | SCAC3 |
| 8 | OUAK | OUR OIL LLC | | | SCAC4 |
| 9 | OUAO | OUGHTON, A & SONS INC | | | SCAC3 |
| 10 | OUBE | OUBRE, IVORY A TRUCKING INC | | | SCAC4 |
| 11 | OUCH | Ouachita R.R. | added 4/1991; 1/1994-7/2005 | | NAKINA |
| 12 | OUCH | Quachita R.R. | added 4/91; 1/94-7/2000 | | NRHS |
| 13 | OUCH | OUACHITA RAILROAD | | | PWRC |
| 14 | OUCH | Ouachita Railroad | | | RSRV |
| 15 | OUCH | OUACHITA RAILROAD | | | SCAC3 |
| 16 | OUCH | OUACHITA RAILROAD | | | SCAC4 |
| 17 | OUCO | ORLANDO UTILITIES COMMISSION | | | SCAC4 |
| 18 | OUCX | Orlando Utilities Commission | added 1/1987; 1/1994-7/2005 | | NAKINA |
| 19 | OUCX | Orlando Utilities Commission | added 1/87; 1/94-7/2000 | | NRHS |
| 20 | OUCX | ORLANDO UTILITIES COMMISSION | | | PWRC |
| 21 | OUCX | Orlando Utilities Commission | | | RSRV |
| 22 | OUCX | ORLANDO UTILITIES COMMISSION | | | SCAC4 |
| 23 | OUCZ | OZARK UTILITIES COMPANY | | | PWRC |
| 24 | OUEL | OUELLET, RAYNOLD TRANSPORT INC | | | SCAC3 |
| 25 | OUEL | OUELLET, RAYNALD | | | SCAC4 |
| 26 | OUEQ | OUELLETTE, LUCIEN, CARTAGE | | | SCAC3 |
| 27 | OUEQ | OUELLETTE, LUCIEN CARTAGE | | | SCAC4 |
| 28 | OUEST | Chemins de fer de l'Ouest | | F | TRAFFIC |
| 29 | OUF | Beijing Eofa International Jet Co., Ltd. | Callsign: Element, | PRC | AVCODES |
| 30 | OUIE | OUI EXPRESS | | | SCAC3 |
| 31 | OUIES | Ouigo España | | E | TRAFFIC |
| 32 | OUIJ | OUIMET TRANSPORT | | | SCAC4 |
| 33 | OUIM | OUIMETTE, ALAN JOHN | | | SCAC4 |
| 34 | OUL | Oklahoma Aviation LLC d/b/a Soulbird | Callsign: Soul Bird, | USA | AVCODES |
| 35 | OUL | Air Atonabee Limited d/b/a City Express | IATA: OU, Callsign: City Express, Designator OU recalled by IATA | CDN | AVCODES |
| 36 | OUL | Air Atonabee | | | ICAO1 |
| 37 | OULF | OUELLETTE FARMS | | | SCAC3 |
| 38 | OULU | OULUN KONTTIVUOKRAUS OY | | FIN | BIC |
| 39 | OUR | Aerotour, S.A. de C.V. | Callsign: Mexitour, | MEX | AVCODES |
| 40 | OURA | OURADA, DANIEL TRUCKING CO | | | SCAC3 |
| 41 | OURA | OUT OF RUINS | | | SCAC4 |
| 42 | OURD | The Ogden Union Ry. and Depot Co. | 1/1975-7/2005 | | NAKINA |
| 43 | OURD | The Ogden Union Ry. and Depot Co. | 1/75-7/2000 | | NRHS |
| 44 | OURD | OGDEN UNION RAILWAY & DEPOT CO. | | | PWRC |
| 45 | OURD | Ogden Union Railway and Depot | | | RSRV |
| 46 | OURD | OGDEN UNION RAILWAY AND DEPOT COMPA | | | SCAC3 |
| 47 | OURK | OUR TRUCKING CO | | | SCAC3 |
| 48 | OURS | OUR SPECIALTY TRANSPORTATION SERVIC | | | SCAC4 |
| 49 | OURT | OUR TRUCK CO | | | SCAC3 |
| 50 | OURW | OUR WAY FREIGHT | | | SCAC3 |
| 51 | OURY | OGDENSBURG BRIDGE & PORT AUTHORITY | | | PWRC |
| 52 | OUS | OGDEN UNION STOCKYARDS COMPANY | | | PWRC |
| 53 | OUSA | ORICA USA INC | | | SCAC4 |
| 54 | OUTB | OUTBOUND EXPRESS INC | | | SCAC3 |
| 55 | OUTI | OWNERS UNITED TRANSPORTATION INC | | | SCAC4 |
| 56 | OUTK | OUR TRUCK CO | | | SCAC3 |
| 57 | OUTP | OUT OF PARADISE GALLERY | | | SCAC4 |
| 58 | OUTT | OUTLAW TRUCKING CO | | | SCAC3 |
| 59 | OUTU | METSO OUTOTEC FINLAND OY | | FIN | BIC |
