| 1 | IO | PT Indonesian Airlines Av iapatria | ICAO: IAA, Callsign: Indo Lines, | RI | AVCODES |
| 2 | IO | Air Paris | Marged into TAT | F | AVCODES |
| 3 | IO | Intercontinental Pacific Airways, Inc. | ICAO: IPC, Callsign: Interpac, IATA designator changed from IS* | RP | AVCODES |
| 4 | IO | Indochina Airlines | ICAO: IDO, Ceased opertions | K | AVCODES |
| 5 | IO | Closed Joint Stock Company IrAero | ICAO: IAE, Callsign: IrAero, | RUS | AVCODES |
| 6 | IOA | Iowa Airways | Callsign: Iowa Air, | USA | AVCODES |
| 7 | IOA | GSEC Monarch and Deccan Aviation Private Limited d/b/a IndiaOne Air | IATA: I7, Callsign: India First, | IND | AVCODES |
| 8 | IOA | Iowa Airways | | | ICAO1 |
| 9 | IOBU | BIOCOTECH AMERICAS INC | | USA | BIC |
| 10 | IOCA | IOC TRUCKING | | | SCAC4 |
| 11 | IOCB | INFINITO INC | | | SCAC4 |
| 12 | IOCC | QUEBEC NORTH SHORE & LABRADOR | | | PWRC |
| 13 | IOCC | INDEPENDENT OIL & COAL COMPANY | | | SCAC4 |
| 14 | IOCM | IRMAS CREATIONS | | | SCAC4 |
| 15 | IOCP | INDEPENDENT OIL COMPANY | | | SCAC4 |
| 16 | IOCR | The Indiana & Ohio Ry. | added 7/2005; 7/2005 | | NAKINA |
| 17 | IOCR | The Indiana & Ohio Central R.R. | added 7/1999; 7/2000; eliminated 7/2005 | | NAKINA |
| 18 | IOCR | The Indiana & Ohio Rail System | eliminated 7/1999 | | NAKINA |
| 19 | IOCR | The Indiana & Ohio Central R.R., Inc. | added 1/1988; 1/1994 | | NAKINA |
| 20 | IOCR | The Indiana & Ohio Central R.R., Inc. | added 1/88; 1/94 | | NRHS |
| 21 | IOCR | The Indiana & Ohio Rail System | eliminated 7/99 | | NRHS |
| 22 | IOCR | The Indiana & Ohio Central R.R. | added 7/99; 7/2000 | | NRHS |
| 23 | IOCR | INDIANA & OHIO RAILWAY [INDIANA & OHIO CENTRAL RAILROAD, INC] | | | PWRC |
| 24 | IOCR | Indiana and Ohio Central Railroad | | | RSRV |
| 25 | IOCT | INTEROCEAN TRADE AND TRANSPORTATION | | | SCAC4 |
| 26 | IOCX | Rail Enterprises, Inc. | added 10/1997; eliminated 4/1998; 7/2000-7/2005 (not listed) | | NAKINA |
| 27 | IOCX | Illinois Oil Co. | 8/1920-1/1945 | | NAKINA |
| 28 | IOCX | Illinois Oil Co. | Apr-38 | | NRHS |
| 29 | IOCX | Rail Enterprises, Inc. | added 10/97; eliminated 4/98; 7/2000 | | NRHS |
| 30 | IOCX | *ILLINOIS OIL CO.. | | | PWRC |
| 31 | IOCX | +RAIL ENTERPRISES INCORPORATED | | | PWRC |
| 32 | IOCX | Illinois Oil Co. (Rail Enterprises) | | | RSRV |
| 33 | IOCX | RAIL ENTERPRISES INC (ACTING AGENT | | | SCAC4 |
| 34 | IOCY | IOC COMPANY LLC | | | SCAC4 |
| 35 | IOE | Island Oil Exploration Limited | Callsign: Island Oil, | STP | AVCODES |
| 36 | IOEP | I O EXPRESS INC | | | SCAC4 |
| 37 | IOER | Indiana & Ohio Rail System | eliminated 10/1994 | | NAKINA |
| 38 | IOER | The Indiana & Ohio Eastern R.R., Inc. | added 1/1988; 1/1994 | | NAKINA |
| 39 | IOER | The Indiana & Ohio Eastern R.R., Inc. | added 1/88; 1/94 | | NRHS |
| 40 | IOER | Indiana & Ohio Rail System | eliminated 10/94 | | NRHS |
| 41 | IOER | RETIRED 01-05-94 ATP (979) [INDIANA & OHIO EASTERN RAILROAD] | | | PWRC |
| 42 | IOER | INTER-ONTARIO EQUIPMENT RENTAL & RE | | | SCAC4 |
| 43 | IOF | INAER Helicopteros Off-Shore, S.A. | Callsign: Inaer Offshore, | E | AVCODES |
| 44 | IOI | Atal Milano S.r.l. | Callsign: Atalmilano, Name changed from Atal. | I | AVCODES |
| 45 | IOI | Atal | Callsign: Atalmilano, Name changed to Atal Milano S.r.l. | I | AVCODES |
| 46 | IOIL | IRVING OIL LIMITED | | | SCAC4 |
| 47 | IOLL | INDEPENDENT OPERATORS LLC | | | SCAC4 |
| 48 | IOLU | INTECH ORGANIC LIMITED | | IND | BIC |
| 49 | IOLU | TEXTAINER EQUIPMENT MANAGEMENT (U.S.) LTD. | | | PWRC |
| 50 | IOLU | Textainer Equipment Management | | | RSRV |
| 51 | IOM | Island Aviation And Travel Ltd. | Callsign: Isle Avia, | GB | AVCODES |
| 52 | IOM | Island Aviation and Travel | | | ICAO1 |
| 53 | IOMC | INTEROCEAN MANAGMENT CORP. | | | SCAC4 |
| 54 | IOMX | IMERYS OILFIELD MINERALS INC | | | SCAC4 |
| 55 | ION | Aire Aviation Ltd. | Callsign: Gorrion, | M | AVCODES |
| 56 | IONA | IONIA MANAGEMENT S A | | | SCAC4 |
| 57 | IONR | ION-TRACE INC | | | SCAC4 |
| 58 | IONY | INDIAGEMS OF NEW YORK INC | | | SCAC4 |
| 59 | IOP | Caliop | | F | AVCODES |
| 60 | IOP | Caliop | Callsign: Caliop, | F | AVCODES |
| 61 | IOPU | CANTREYN, LTD | | | SCAC3 |
| 62 | IOPX | MidAmerican Energy Co. | added 4/1996; 7/2000-7/2005 | | NAKINA |
| 63 | IOPX | Midwest Power Systems | added 4/1993; 1/1994; eliminated 4/1996 | | NAKINA |
| 64 | IOPX | Iowa Power & Light Co. | added 4/1977; 1/1980; eliminated 4/1993 | | NAKINA |
| 65 | IOPX | Iowa Power & Light Co. | 1/80; eliminated 4/93 | | NRHS |
| 66 | IOPX | Midwest Power Systems | added 4/93; 1/94; eliminated 4/96 | | NRHS |
| 67 | IOPX | MidAmerican Energy Co. | added 4/96; 7/2000 | | NRHS |
| 68 | IOPX | MIDAMERICAN ENERGY CO [MIDWEST POWER SYSTEMS; IOWA POWER & LIGHT] | | | PWRC |
| 69 | IOPX | Iowa Power and Light (MidAmerican Energy) | | | RSRV |
| 70 | IOPZ | IOWA PUBLIC SERVICE | | | PWRC |
| 71 | IORX | Indiahoma Refining Co. | 8/1920-7/1923 | | NAKINA |
| 72 | IORX | *INDIAHOMA REFINING CO. | | | PWRC |
| 73 | IORX | *INDIANA & OHIO RAILWAY - PASSENGER SVC. | | | PWRC |
| 74 | IORY | The Indiana & Ohio Ry. | added 7/1999; 7/2000-7/2005 | | NAKINA |
| 75 | IORY | The Indiana & Ohio Rail System | eliminated 7/1999 | | NAKINA |
| 76 | IORY | The Indiana & Ohio Ry. Co. | added 1/1988; 1/1994 | | NAKINA |
| 77 | IORY | The Indiana & Ohio Ry. Co. | added 1/88; 1/94 | | NRHS |
| 78 | IORY | The Indiana & Ohio Rail System | eliminated 7/99 | | NRHS |
| 79 | IORY | The Indiana & Ohio Ry. | added 7/99; 7/2000 | | NRHS |
| 80 | IORY | INDIANA & OHIO RAILWAY COMPANY | | | PWRC |
| 81 | IORY | Indiana and Ohio Railway | | | RSRV |
| 82 | IORY | INDIANA & OHIO RAILWAY COMPANY | | | SCAC4 |
| 83 | IORY | Indiana & Ohio Railway Company | | USA | TRAFFIC |
| 84 | IOS | Isles of Scilly Skybus Ltd. | IATA: 5Y, Callsign: Scillonia, Designator 5Y* recalled by IATA. IATA designator changed from FW | GB | AVCODES |
| 85 | IOS | Isles of Scilly Skybus Ltd. | IATA: FW, Callsign: Scillonia, IATA designator changed to 5Y* | GB | AVCODES |
| 86 | IOS | Isles of Scilly Skybus Ltd. | Callsign: Scillonia, Designator 5Y* recalled by IATA. IATA designator changed from FW | GB | AVCODES |
| 87 | IOS | Isles of Scilly Skybus | | | ICAO1 |
| 88 | IOSC | INTEROCEAN STEAMSHIP CORPORATION, S | | | SCAC4 |
| 89 | IOSF | IOSCO TRUCKING INC | | | SCAC4 |
| 90 | IOSX | Interstate Oil Shippers Line | 7/1923 | | NAKINA |
| 91 | IOSX | *INTERSTATE OIL SHIPPERS LINE | | | PWRC |
| 92 | IOSX | *INDIANA & OHIO SCENIC RAIL CORP. | | | PWRC |
| 93 | IOTA | IOTA | | | SCAC4 |
| 94 | IOTC | INTERNATIONAL ORDNANCE TECHNOLOGIES | | | SCAC4 |
| 95 | IOTG | IBISEVIC, OMER TRUCKING | | | SCAC4 |
| 96 | IOTK | INDENT OILFIELD TRUCKING | | | SCAC4 |
| 97 | IOTL | I O TRUCKING LLC | | | SCAC4 |
| 98 | IOTN | INDIAN ORCHARD TRUCKING INC | | | SCAC4 |
| 99 | IOTP | IN AND OUT TRANSPORT | | | SCAC4 |
| 100 | IOTX | Inter Ocean Refining Co. | 8/1920-7/1923 | | NAKINA |
