| 501 | OCLR | OCEANCOLOUR SHIPPING LTD. | | | SCAC3 |
| 502 | OCLR | ORGANIC CONNECTIONS LTD | | | SCAC4 |
| 503 | OCLT | OCEANLINK INCORPORATED | | | SCAC4 |
| 504 | OCLU | MAERSK A/S | | DK | BIC |
| 505 | OCLU | OVERSEAS CONTAINERS, LTD. | | | PWRC |
| 506 | OCLU | Overseas Containers | | | RSRV |
| 507 | OCLU | OVERSEAS CONTAINER LTD | | | SCAC3 |
| 508 | OCLU | P & O NEDLLOYD BV | | | SCAC4 |
| 509 | OCM | O'Connor Airlines | IATA: UQ, Callsign: O'Connor, Name changed from O'Connor-Mt. Gambier's Airline | AUS | AVCODES |
| 510 | OCM | O'Connor-Mt. Gambier's Airline | IATA: UQ, Callsign: O'Connor, Name changed to O'Connor Airlines | AUS | AVCODES |
| 511 | OCMD | OCEAN CITY, MAYOR AND CITY COUNCIL | | | SCAC3 |
| 512 | OCME | OAK CLIFF METALS | | | SCAC4 |
| 513 | OCMF | ONE CALL MOTOR FREIGHT INC | | | SCAC3 |
| 514 | OCMG | ORANGE COAST MOVING SYSTEMS INC | | | SCAC3 |
| 515 | OCMP | OCEANIC COMP LTD | | | SCAC3 |
| 516 | OCMP | OCEANIC COMP LTD | | | SCAC4 |
| 517 | OCMR | OCEANMARIS MANAGEMENT INC | | | SCAC3 |
| 518 | OCMS | OXBOW CARBON & MINERALS LLC | | | SCAC4 |
| 519 | OCMU | OC Container | | | RSRV |
| 520 | OCMU | OC CONTAINER CORP | | | SCAC4 |
| 521 | OCN | Octavia Airlines | IATA: O4, Callsign: Octaflight, Ceased operations. | F | AVCODES |
| 522 | OCN | Ocean GmbH | Callsign: Ocean, Subsidiary of Lufthansa. Name changed to EW Discover GmbH. | D | AVCODES |
| 523 | OCN | EW Discover GmbH t/a Eurowings Discover | IATA: 4Y, Callsign: Ocean, Subsidiary of Lufthansa. Name changed from Ocean GmbH. | D | AVCODES |
| 524 | OCN | O Air | | | ICAO1 |
| 525 | OCN | OLDE COLONY & NEW PORT | | | PWRC |
| 526 | OCNA | OCEAN AIR INTERNATIONAL INC | | | SCAC3 |
| 527 | OCNC | O C & CO | | | SCAC3 |
| 528 | OCNE | OCONEE TRANSFER SERVICE INC | | | SCAC3 |
| 529 | OCNE | OCEAN KAYAK | | | SCAC4 |
| 530 | OCNF | OCEAN-5 EXPRESS LINE INC. | | | SCAC3 |
| 531 | OCNH | OCEANANCHOR MARITIME LIMITED | | | SCAC4 |
| 532 | OCNI | OCONNOR PETROLEUM TRUCKING INC | | | SCAC3 |
| 533 | OCNJ | OIL CITY PETROLEUM | | | SCAC4 |
| 534 | OCNK | OCEAN KNIGHT SHIPPING INC | | | SCAC4 |
| 535 | OCNM | OCONNELLS MOVING SERVICE | | | SCAC3 |
| 536 | OCNM | ONTARIO CABINET MAKERS | | | SCAC4 |
| 537 | OCNO | OCONNOR CHARTER & TOURS SERVICE | | | SCAC3 |
| 538 | OCNQ | OCEANSIDE HARBOR TRANSFER CO INC | | | SCAC3 |
| 539 | OCNR | OCONNER TRUCKING CO | | | SCAC3 |
| 540 | OCNR | OCEANARIUM SHIPPING LIMITED | | | SCAC4 |
| 541 | OCNS | OCEANIC CONTAINER SERVICE INC | | | SCAC3 |
| 542 | OCNS | OCEANPORT LLC | | | SCAC4 |
| 543 | OCNT | OCONNOR, THOMAS V TRUCKING | | | SCAC3 |
| 544 | OCNU | OCEANIC CONTAINERS, LLC | | USA | BIC |
| 545 | OCNU | OCEAN LINES INC. | | | PWRC |
| 546 | OCNU | Ocean Lines | | | RSRV |
| 547 | OCNU | OCEAN LINE INC | | | SCAC3 |
| 548 | OCNU | OCEAN LINE INC | | | SCAC4 |
| 549 | OCNW | OCEANWAY TRANSPORT INC | | | SCAC3 |
| 550 | OCO | Ostend Air College | Callsign: Air College, | B | AVCODES |
| 551 | OCO | Ostend Air College | | | ICAO1 |
| 552 | OCOB | OCONNOR BROTHERS TRUCKING COMPANY | | | SCAC3 |
| 553 | OCOI | OSCO INCORPORATED | | | SCAC3 |
| 554 | OCOI | OSCO INCORPORATED | | | SCAC4 |
| 555 | OCOL | OCONNOR OILFIELD SERVICE | | | SCAC3 |
| 556 | OCOP | OCEAN OPERATORS INC | | | SCAC3 |
| 557 | OCOR | OPEX CORP | | | SCAC4 |
| 558 | OCOT | OCONNOR TRUCKING COMPANY, INCORPORA | | | SCAC3 |
| 559 | OCOU | AONOCO LIMITED | | HK | BIC |
| 560 | OCOX | O'Bannon Co. | 7/1923 | | NAKINA |
| 561 | OCOX | O'Bannon Corp. | 8/1920 | | NAKINA |
| 562 | OCOX | O'BANNON CO. | | | PWRC |
| 563 | OCOZ | IWENS CORNING | | | PWRC |
| 564 | OCPU | OCEAN CLOUD BV | | B | BIC |
| 565 | OCPU | OVERSEAS CONTAINER LTD | | | SCAC3 |
| 566 | OCPU | OVERSEAS CONTAINER LTD | | | SCAC4 |
| 567 | OCPV | ORANGEVILLE CONCRETE PUMPING SERVIC | | | SCAC4 |
| 568 | OCPX | Oxy Vinyls L.P. | added 7/1999; 7/2000-7/2005 | | NAKINA |
| 569 | OCPX | Occidental Chemical Corp. | added 1/1992; 1/1994; eliminated 7/1999 | | NAKINA |
| 570 | OCPX | Occidental Chemical Corp., PVC Resins | added 1/1988; eliminated 1/1992 | | NAKINA |
| 571 | OCPX | Occidental Chemical Corp., PVC Resins | added 1/88; eliminated 1/92 | | NRHS |
| 572 | OCPX | Occidental Chemical Corp. | added 1/92; 1/94; eliminated 7/99 | | NRHS |
| 573 | OCPX | Oxy Vinyls L.P. | added 7/99; 7/2000 | | NRHS |
| 574 | OCPX | OCCIDENTAL CHEMICAL CORP., PVC RESINS (OXY VINYLS) | | | PWRC |
| 575 | OCPX | Occidental Chemical (Oxy Vinyls) | | | RSRV |
| 576 | OCPX | OXY VINYLS LP | | | SCAC4 |
| 577 | OCR | Ocean Air PVT Ltd. | Callsign: Ocean Air, Ceased operations | MV | AVCODES |
| 578 | OCR | Oklahoma Central R.R. Co. | added 1/1988; eliminated 7/1990; 1/1994-7/2005 | | NAKINA |
| 579 | OCR | Oklahoma Central R.R. Co. | added 1/88; eliminated 7/90; 1/94-7/2000 | | NRHS |
| 580 | OCR | OKLAHOMA CENTRAL RAILROAD COMPANY | | | PWRC |
| 581 | OCR | Oklahoma Central Railroad | | | RSRV |
| 582 | OCR | OKLAHOMA CENTRAL RAILROAD CO | | | SCAC4 |
| 583 | OCRE | ORTEGA, CRESENCIO | | | SCAC4 |
| 584 | OCRI | OCEAN AIR TRANSPORTATION INC | | | SCAC4 |
| 585 | OCRM | Oak Capital Rail Management GmbH | | DE | VKM |
| 586 | OCRR | Ottawa Central Ry. | added 4/2000; 7/2000-7/2005 | | NAKINA |
| 587 | OCRR | Ottawa Central Ry. | added 4/2000; 7/2000 | | NRHS |
| 588 | OCRR | OTTAWA CENTRAL RAILWAY | | | PWRC |
| 589 | OCRR | Ottawa Central Railway | | | RSRV |
| 590 | OCRR | OTTAWA CENTRAL RAILWAY | | | SCAC4 |
| 591 | OCRR | Ottawa Central Ry. | | CDN | TRAFFIC |
| 592 | OCRS | OIL CARRIERS SHIPPING LIMITED | | | SCAC3 |
| 593 | OCRS | OCEAN RUN SEAFOOD CANADA LTD | | | SCAC4 |
| 594 | OCRV | OCOEE RIVER TRANSPORT INC | | | SCAC3 |
| 595 | OCRX | Oklahoma Central R.R. | 7/2000-7/2005 (not listed) | | NAKINA |
| 596 | OCRX | Oil Creek Refining Co. | 7/1923-7/1935 | | NAKINA |
| 597 | OCRX | Oklahoma Central R.R. | Jul-00 | | NRHS |
| 598 | OCRX | *OIL CREEK REFINING CO. | | | PWRC |
| 599 | OCRX | +OKLAHOMA CENTRAL RAILROAD | | | PWRC |
| 600 | OCRX | Oil Creek Refining | | | RSRV |
