| 1 | OI | Aspiring Air Limited | Designator OI* reinstated by IATA. Founded in 1974 | NZ | AVCODES |
| 2 | OI | Heli Transport SA | ICAO: HLT, Callsign: Heli Transport, | F | AVCODES |
| 3 | OI | Slov-Air | | CS | AVCODES |
| 4 | OI | H. Jonsson air Taxi | Callsign: Odinn, | USA | AVCODES |
| 5 | OI | Italia Tour Airlines S.p.A. | ICAO: IAZ, AOC license was suspended by ENAC on 30 November 2009. | I | AVCODES |
| 6 | OI | Hinterland Aviation Pty Ltd | ICAO: HND, Callsign: Hinterland, | AUS | AVCODES |
| 7 | OI | Okapi Airways | ICAO: OKP, Callsign: Okapi, | ZRE | AVCODES |
| 8 | OIAB | ONE THOUSAND (1000) ISLAND AIRBOATS | | | SCAC4 |
| 9 | OIAG | OIA GLOBAL LOGISTICS | | | SCAC4 |
| 10 | OIBT | OUT AND IN BOAT TRANSIT INC | | | SCAC3 |
| 11 | OIC | Iwamoto Crane Co Ltd | | | ICAO1 |
| 12 | OIDZ | OIL DRY | | | PWRC |
| 13 | OIER | The Indiana & Ohio Rail System | eliminated 7/1994 | | NAKINA |
| 14 | OIER | The Indiana & Ohio Rail System | eliminated 7/94 | | NRHS |
| 15 | OIER | INDIANA & OHIO RAIL SYSTEM | | | PWRC |
| 16 | OIER | OFFICIAL INTERMODAL EQUIPMENT REGIS | | | SCAC3 |
| 17 | OIFI | ORE-IDA FO0DS INC | | | SCAC3 |
| 18 | OIFS | OILFIELD SERVICES INC | | | SCAC3 |
| 19 | OIG | Oigawa Railway | | J | TRAFFIC |
| 20 | OIIC | OPTIBLEND INDUSTRIES INC | | | SCAC4 |
| 21 | OILC | OIL CORPORATION | | | SCAC4 |
| 22 | OILCO | OILCO KDA Skopie, R Macedonia | | BG | VKM |
| 23 | OILE | OIL EXPRESS, INC | | | SCAC3 |
| 24 | OILF | OILFIELD EXPRESS | | | SCAC3 |
| 25 | OILI | OIL TRANSPORT CO INC | | | SCAC3 |
| 26 | OILI | OHLSON INTERNATIONAL LOGISTICS INCO | | | SCAC4 |
| 27 | OILJ | ONPOINT INTERNATIONAL LLC | | | SCAC4 |
| 28 | OILL | OIL TANK LINE INC | | | SCAC3 |
| 29 | OILL | ON TIME FREIGHT LTD | | | SCAC4 |
| 30 | OILM | OIL MAN CORPORATION, THE | | | SCAC4 |
| 31 | OILO | OWENS-ILLINOIS | | | SCAC3 |
| 32 | OILQ | OIL TRANSPORT INC | | | SCAC3 |
| 33 | OILS | OIL SUPPLY CORP | | | SCAC4 |
| 34 | OILT | OIL TRANSPORT COMPANY | | | SCAC3 |
| 35 | OILT | OIL CAPITAL TRANSPORTATION LLC | | | SCAC4 |
| 36 | OILT | Oil Terminal Constanta | | RO | TRAFFIC |
| 37 | OILU | OIL2U PTY LTD | | AUS | BIC |
| 38 | OILW | OLAM INTERNATIONAL LIMITED | | | SCAC4 |
| 39 | OILX | General Electric Railcar Services Corp. | added 1/1984; 1/1994; eliminated 10/1994 | | NAKINA |
| 40 | OILX | North American Car Corp. | eliminated 1/1984 | | NAKINA |
| 41 | OILX | Transcontinental Transport, Inc. | added 10/1960; 4/1961; eliminated 7/1962 | | NAKINA |
| 42 | OILX | Oil Tank Line Co. | 7/1937-10/1957; eliminated 10/1960 | | NAKINA |
| 43 | OILX | Oil Tank Line Co. | 4/38-10/57; eliminated 10/60 | | NRHS |
| 44 | OILX | Transcontinental Transport, Inc. | added 10/60; 4/61 | | NRHS |
| 45 | OILX | General Electric Railcar Services Corp. | 1/94; eliminated 10/94 | | NRHS |
| 46 | OILX | *TRANSCONTINENTAL TRANSPORT [OIL TANK LINE CO.] | | | PWRC |
| 47 | OILX | *(NATX) GENERAL ELECTRIC RAILCAR SERVICES CORPORATION | | | PWRC |
| 48 | OILX | GE Rail Services | | | RSRV |
| 49 | OIM | Oliver Iron Mining | | USA | TRAFFIC |
| 50 | OIMC | OUTER ISLANDS MARINE CORPORATION | | | SCAC3 |
| 51 | OIML | ORION INTERNATIONAL MEGA-LOGISTICS | | | SCAC4 |
| 52 | OIMT | OIM TRANSIT CORP | | | PWRC |
| 53 | OINC | OFS INC | | | SCAC4 |
| 54 | OINS | Avirunion, a.s., závod Nové Sedlo | | CZ | VKM |
| 55 | OIPC | OIL PRODUCTS CO | | | SCAC3 |
| 56 | OIR | Slov Air J.S.C | Callsign: Slovair, | SK | AVCODES |
| 57 | OIRC | OILFIELD REQUIREMENTS CO & O R C TR | | | SCAC3 |
| 58 | OIRH | Avirunion, a.s., závod Rudolfova huť | | CZ | VKM |
| 59 | OISC | OIL SERVICES CO | | | SCAC3 |
| 60 | OISL | OAK INTERMODAL SERVICES LLC | | | SCAC4 |
| 61 | OISR | ONE THOUSAND (1000) ISLANDS SALES - | | | SCAC4 |
| 62 | OISV | OIL SERVICES INC | | | SCAC3 |
| 63 | OISV | ONYX INDUSTRIAL SERVICES INC | | | SCAC4 |
| 64 | OITF | OILFIELDS TRUCKING COMPANY | | | SCAC3 |
| 65 | OITO | OVERSEAS INTERNATIONAL TOURS INC | | | SCAC3 |
| 66 | OITS | OIL TRANSPORT SHIPPING LIMITED | | | SCAC3 |
| 67 | OIUA | OILFIELD INSTRUMENTATION USA | | | SCAC4 |
| 68 | OIX | Orion-X | Callsign: Orionx, | RUS | AVCODES |
| 69 | OIX | Orion-x | | | ICAO1 |
| 70 | OIXI | OIX INCORPORATED | | | SCAC4 |
