| 1401 | OHSE | OHIO STEEL EXPRESS INC | | | SCAC3 |
| 1402 | OHSE | OKANAGAN HOT SHOT ENTERPRISES LTD | | | SCAC4 |
| 1403 | OHSG | OHIO STANDARD MILEAGE AND/OR RATE F | | | SCAC3 |
| 1404 | OHSH | OHARE SHUTTLE INC | | | SCAC3 |
| 1405 | OHSH | OFFSHORE HEAVY TRANSPORT AS | | | SCAC4 |
| 1406 | OHSI | OHIO SWIFTWAY INC | | | SCAC3 |
| 1407 | OHSO | OSHCO INC | | | SCAC3 |
| 1408 | OHSQ | OZBURN HESSEY DISTRIBUTION CO | | | SCAC3 |
| 1409 | OHSS | OHIO SECURITY SYSTEMS INC | | | SCAC3 |
| 1410 | OHST | OHSE TRANSPORTATION INC | | | SCAC3 |
| 1411 | OHSV | OHARE SERVICES INC | | | SCAC3 |
| 1412 | OHSW | OHSWALDT, RAYMOND | | | SCAC3 |
| 1413 | OHSX | Schoenwald Petroleum Co. | 1/1946-1/1950 | | NAKINA |
| 1414 | OHSX | O.H. Schoenwald | 1/1943-1/1945 | | NAKINA |
| 1415 | OHSX | SCHOENWALD PETROLEUM CO. [O. H. SCHOENWALD] | | | PWRC |
| 1416 | OHTA | ONE HORN TRANSPORTATION INC | | | SCAC4 |
| 1417 | OHTH | OPHARDT HYGIENE TECHNOLOGIES INC | | | SCAC4 |
| 1418 | OHTI | OLE HICKORY TRANSPORT INC | | | SCAC3 |
| 1419 | OHTN | OVERLAND HORSE TRANSPORTATION | | | SCAC3 |
| 1420 | OHTP | OHIO TRANSPORT CORPORATION INC | | | SCAC3 |
| 1421 | OHTP | OHIO TRANSPORT CORPORATION INC | | | SCAC4 |
| 1422 | OHTQ | OHIO TRUCK SERVICE INC | | | SCAC3 |
| 1423 | OHTR | O & H TRUCKING | | | SCAC3 |
| 1424 | OHTR | OHIO TIRE TRANSPORTATION | | | SCAC4 |
| 1425 | OHTW | O'HARE TOWING | | | SCAC4 |
| 1426 | OHTX | O&H Transport, Inc. | added 10/1967; 4/1970-1/1980 | | NAKINA |
| 1427 | OHTX | O&H Transport, Inc. | added 10/67; 4/70-1/80 | | NRHS |
| 1428 | OHTX | O & h TRANSPORT, INC. | | | PWRC |
| 1429 | OHVD | OHIO VALLEY DISTRUBUTION | | | SCAC3 |
| 1430 | OHVE | OHIO VALLEY EXPEDITING | | | SCAC4 |
| 1431 | OHVO | OHIO VALLEY TOWING INC | | | SCAC3 |
| 1432 | OHVR | OHIO VALLEY RELOAD INC | | | SCAC3 |
| 1433 | OHVT | OHIO VALLEY TRANSPORT INC | | | SCAC3 |
| 1434 | OHWL | O'HARE WISCONSIN LIMOUSINE SERVICE | | | SCAC4 |
| 1435 | OHWT | OHIO WAREHOUSING TRANSFER CO INC | | | SCAC3 |
| 1436 | OHY | Onur Air Tasimacilik A/S | IATA: 8Q, Callsign: Onur Air, Suspended operations in December 2021. Filed for Bankruptcy in April 2022. | TR | AVCODES |
| 1437 | OHY | Onur Air | IATA: 8Q | | ICAO1 |
| 1438 | OHYU | Mitshubishi Ohyubari Railway | | J | TRAFFIC |
| 1439 | OI | Aspiring Air Limited | Designator OI* reinstated by IATA. Founded in 1974 | NZ | AVCODES |
| 1440 | OI | Heli Transport SA | ICAO: HLT, Callsign: Heli Transport, | F | AVCODES |
| 1441 | OI | Slov-Air | | CS | AVCODES |
| 1442 | OI | H. Jonsson air Taxi | Callsign: Odinn, | USA | AVCODES |
| 1443 | OI | Italia Tour Airlines S.p.A. | ICAO: IAZ, AOC license was suspended by ENAC on 30 November 2009. | I | AVCODES |
| 1444 | OI | Hinterland Aviation Pty Ltd | ICAO: HND, Callsign: Hinterland, | AUS | AVCODES |
| 1445 | OI | Okapi Airways | ICAO: OKP, Callsign: Okapi, | ZRE | AVCODES |
| 1446 | OIAB | ONE THOUSAND (1000) ISLAND AIRBOATS | | | SCAC4 |
| 1447 | OIAG | OIA GLOBAL LOGISTICS | | | SCAC4 |
| 1448 | OIBT | OUT AND IN BOAT TRANSIT INC | | | SCAC3 |
| 1449 | OIC | Iwamoto Crane Co Ltd | | | ICAO1 |
| 1450 | OIDZ | OIL DRY | | | PWRC |
| 1451 | OIER | The Indiana & Ohio Rail System | eliminated 7/1994 | | NAKINA |
| 1452 | OIER | The Indiana & Ohio Rail System | eliminated 7/94 | | NRHS |
| 1453 | OIER | INDIANA & OHIO RAIL SYSTEM | | | PWRC |
| 1454 | OIER | OFFICIAL INTERMODAL EQUIPMENT REGIS | | | SCAC3 |
| 1455 | OIFI | ORE-IDA FO0DS INC | | | SCAC3 |
| 1456 | OIFS | OILFIELD SERVICES INC | | | SCAC3 |
| 1457 | OIG | Oigawa Railway | | J | TRAFFIC |
| 1458 | OIIC | OPTIBLEND INDUSTRIES INC | | | SCAC4 |
| 1459 | OILC | OIL CORPORATION | | | SCAC4 |
| 1460 | OILCO | OILCO KDA Skopie, R Macedonia | | BG | VKM |
| 1461 | OILE | OIL EXPRESS, INC | | | SCAC3 |
| 1462 | OILF | OILFIELD EXPRESS | | | SCAC3 |
| 1463 | OILI | OIL TRANSPORT CO INC | | | SCAC3 |
| 1464 | OILI | OHLSON INTERNATIONAL LOGISTICS INCO | | | SCAC4 |
| 1465 | OILJ | ONPOINT INTERNATIONAL LLC | | | SCAC4 |
| 1466 | OILL | OIL TANK LINE INC | | | SCAC3 |
| 1467 | OILL | ON TIME FREIGHT LTD | | | SCAC4 |
| 1468 | OILM | OIL MAN CORPORATION, THE | | | SCAC4 |
| 1469 | OILO | OWENS-ILLINOIS | | | SCAC3 |
| 1470 | OILQ | OIL TRANSPORT INC | | | SCAC3 |
| 1471 | OILS | OIL SUPPLY CORP | | | SCAC4 |
| 1472 | OILT | OIL TRANSPORT COMPANY | | | SCAC3 |
| 1473 | OILT | OIL CAPITAL TRANSPORTATION LLC | | | SCAC4 |
| 1474 | OILT | Oil Terminal Constanta | | RO | TRAFFIC |
| 1475 | OILU | OIL2U PTY LTD | | AUS | BIC |
| 1476 | OILW | OLAM INTERNATIONAL LIMITED | | | SCAC4 |
| 1477 | OILX | General Electric Railcar Services Corp. | added 1/1984; 1/1994; eliminated 10/1994 | | NAKINA |
| 1478 | OILX | North American Car Corp. | eliminated 1/1984 | | NAKINA |
| 1479 | OILX | Transcontinental Transport, Inc. | added 10/1960; 4/1961; eliminated 7/1962 | | NAKINA |
| 1480 | OILX | Oil Tank Line Co. | 7/1937-10/1957; eliminated 10/1960 | | NAKINA |
| 1481 | OILX | Oil Tank Line Co. | 4/38-10/57; eliminated 10/60 | | NRHS |
| 1482 | OILX | Transcontinental Transport, Inc. | added 10/60; 4/61 | | NRHS |
| 1483 | OILX | General Electric Railcar Services Corp. | 1/94; eliminated 10/94 | | NRHS |
| 1484 | OILX | *TRANSCONTINENTAL TRANSPORT [OIL TANK LINE CO.] | | | PWRC |
| 1485 | OILX | *(NATX) GENERAL ELECTRIC RAILCAR SERVICES CORPORATION | | | PWRC |
| 1486 | OILX | GE Rail Services | | | RSRV |
| 1487 | OIM | Oliver Iron Mining | | USA | TRAFFIC |
| 1488 | OIMC | OUTER ISLANDS MARINE CORPORATION | | | SCAC3 |
| 1489 | OIML | ORION INTERNATIONAL MEGA-LOGISTICS | | | SCAC4 |
| 1490 | OIMT | OIM TRANSIT CORP | | | PWRC |
| 1491 | OINC | OFS INC | | | SCAC4 |
| 1492 | OINS | Avirunion, a.s., závod Nové Sedlo | | CZ | VKM |
| 1493 | OIPC | OIL PRODUCTS CO | | | SCAC3 |
| 1494 | OIR | Slov Air J.S.C | Callsign: Slovair, | SK | AVCODES |
| 1495 | OIRC | OILFIELD REQUIREMENTS CO & O R C TR | | | SCAC3 |
| 1496 | OIRH | Avirunion, a.s., závod Rudolfova huť | | CZ | VKM |
| 1497 | OISC | OIL SERVICES CO | | | SCAC3 |
| 1498 | OISL | OAK INTERMODAL SERVICES LLC | | | SCAC4 |
| 1499 | OISR | ONE THOUSAND (1000) ISLANDS SALES - | | | SCAC4 |
| 1500 | OISV | OIL SERVICES INC | | | SCAC3 |
