| 2501 | ONTR | ONEILL TRANSFER INC | | | SCAC3 |
| 2502 | ONTR | ONEILL TRANSFER INC | | | SCAC4 |
| 2503 | ONTS | ONE THREE SEVEN LARK STREET CORP (1 | | | SCAC3 |
| 2504 | ONTT | ON TIME TRUCKLINES INC | | | SCAC3 |
| 2505 | ONTU | NEW ONTO SHIPPING INTERNATIONAL | | | SCAC4 |
| 2506 | ONTV | ON TIME MESSENGER SERVICE, INC | | | SCAC3 |
| 2507 | ONTW | OCEAN NAVIGATION 2 CO LTD | | | SCAC4 |
| 2508 | ONTX | On-Track Railcar Services, Inc. | added 4/1988; 1/1994-7/2005 | | NAKINA |
| 2509 | ONTX | On-Track Railcar Services, Inc. | added 4/88; 1/94-7/2000 | | NRHS |
| 2510 | ONTX | ON-TRACK RAILCAR SERVICES, INC. | | | PWRC |
| 2511 | ONTX | On-Track Railcar Services | | | RSRV |
| 2512 | ONTX | ON-TRACK RAILCAR SERVICES, INC. | | | SCAC4 |
| 2513 | ONVA | OXL N V | | | SCAC4 |
| 2514 | ONVL | ONIRATO NAVIGATION LTD | | | SCAC4 |
| 2515 | ONW | Oregon & Northwestern R.R. Co. | 1/1975; added 4/1979; 1/1980; eliminated 4/1989; eliminated 4/1990 | | NAKINA |
| 2516 | ONW | Oregon & Northwestern R.R. Co. | 1/75; added 4/79; 1/80; eliminated 4/89; eliminated 4/90 | | NRHS |
| 2517 | ONW | OREGON & NORTHWESTERN RAILROAD CO. | | | PWRC |
| 2518 | ONW | Oregon and Northwestern Railroad | | | RSRV |
| 2519 | ONWC | ON WHEEL CARRIER | | | SCAC4 |
| 2520 | ONWH | ONEILL & WHITAKER INC | | | SCAC3 |
| 2521 | ONWH | ONEIDA WAREHOUSING LLC | | | SCAC4 |
| 2522 | ONWQ | O & W TRUCK LINES | | | SCAC3 |
| 2523 | ONWR | Oregon & Northwestern Railroad | | USA | TRAFFIC |
| 2524 | ONWY | ONE WAY TRANSPORTATION INC | | | SCAC4 |
| 2525 | ONX | Closed Joint-Stock Company Air Onix Airlines | IATA: OG, Callsign: Onixair, Ceased operations on 12 December 2013. Incorporated on 27 June 2007 | UA | AVCODES |
| 2526 | ONXD | OLAN TRANSPORT LTD | | | SCAC4 |
| 2527 | ONXE | OHIO NATIONAL EXPRESS | | | SCAC4 |
| 2528 | ONXR | ONYX TRANSPORTATION INC | | | SCAC3 |
| 2529 | ONXT | ONYX TOURS AND CHARTERS | | | SCAC3 |
| 2530 | ONY | 25Only Aviation GmbH | IATA: 4Q, Callsign: Only Jet, | D | AVCODES |
| 2531 | ONYQ | OVERMYER DISTRIBUTION SERVICES INC | | | SCAC3 |
| 2532 | ONYS | ORIGINAL NEW YORK SELTZER | | | SCAC3 |
| 2533 | ONYS | OSNAYA ALEGRIA, AURELIO | | | SCAC4 |
| 2534 | ONYW | ONLYWAY TRANSPORTATION INC | | | SCAC3 |
| 2535 | ONYX | American Colloid Co. | 4/1970; added 7/1970; 1/1975-1/1980; eliminated 1/1985; eliminated 7/1985 | | NAKINA |
| 2536 | ONYX | American Colloid Co. | 4/70; added 7/70; 1/75-1/80; eliminated 1/85 | | NRHS |
| 2537 | ONYX | AMERICAN COLLOID COMPANY | | | PWRC |
| 2538 | ONYX | American Colloid | | | RSRV |
| 2539 | OO | SkyWest Airlines | ICAO: SKW, Callsign: Skywest, IATA Prefix: 302, Name changed from Sky West Airlines | USA | AVCODES |
| 2540 | OO | Sky West Airlines | ICAO: SKW, Callsign: Skywest, IATA Prefix: 302, Name changed to SkyWest Airlines | USA | AVCODES |
| 2541 | OOAK | ONE OF A KIND TOURS INC | | | SCAC3 |
| 2542 | OOAK | OOSTLANDER, JEANNETTE | | | SCAC4 |
| 2543 | OOCC | OOCL (CANADA) INC | | | SCAC3 |
| 2544 | OOCC | OOCL (CANADA) INC | | | SCAC4 |
| 2545 | OOCL | ORIENT OVERSEAS CONTAINER LINE, INC. | | | PWRC |
| 2546 | OOCL | ORIENT OVERSEAS CONTAINER LINE, INC | | | SCAC3 |
| 2547 | OOCO | OLIVER OIL CO INC | | | SCAC3 |
| 2548 | OOCO | OOCL LOGISTICS LINE LIMITED | | | SCAC4 |
| 2549 | OOCU | ORIENT OVERSEAS CONTAINER LINE LTD. | | HK | BIC |
| 2550 | OOCZ | ORIENT OVERSEAS CONTAINER LINE, INC. | | | PWRC |
| 2551 | OOCZ | Orient Overseas Line | | | RSRV |
| 2552 | OOCZ | ORIENT OVERSEAS LINE, INC. | | | SCAC4 |
| 2553 | OOE | Air Operations of Europe | Callsign: Air Ops, Ceased operations in 1996. | S | AVCODES |
| 2554 | OOEI | OHIO ONTARIO EXPRESS INC | | | SCAC3 |
| 2555 | OOEI | OHIO ONTARIO EXPRESS INC | | | SCAC4 |
| 2556 | OOFA | ONE ONE FOUR TWO NINE EIGHT FIVE (1 | | | SCAC4 |
| 2557 | OOFD | ONE ONE FOUR THREE SEVEN SIX | | | SCAC4 |
| 2558 | OOFS | ONE ONE FIVE SIX SEVEN TWO NINE (11 | | | SCAC4 |
| 2559 | OOGC | ORMOND OIL AND GAS COMPANY INC | | | SCAC4 |
| 2560 | OOIC | OWNERS & OPERATORS INC | | | SCAC4 |
| 2561 | OOIL | ORANGE OIL CO INC | | | SCAC4 |
| 2562 | OOKG | OMARO TRUCKING | | | SCAC4 |
| 2563 | OOLC | ORIENT OVERSEAS CONTAINER LINE, INC. | | | PWRC |
| 2564 | OOLC | ORIENT OVERSEAS CONTAINER LINE, INC | | | SCAC3 |
| 2565 | OOLG | OPTION ONE LOGISTICS | | | SCAC4 |
| 2566 | OOLL | ORIENT OVERSEAS CONTAINER LINE LIMI | | | SCAC4 |
| 2567 | OOLU | ORIENT OVERSEAS CONTAINER LINE LTD. | | HK | BIC |
| 2568 | OOLU | ORIENT OVERSEAS CONTAINER LINE, INC. | | | PWRC |
| 2569 | OOLU | Orient Overseas Container Line | | | RSRV |
| 2570 | OOLU | ORIENT OVERSEAS CONTAINER LINE INC | | | SCAC3 |
| 2571 | OOLU | ORIENT OVERSEAS CONTAINER LINE LTD | | | SCAC4 |
| 2572 | OOLY | OKUNFULURE, OLUBIYISOLA | | | SCAC4 |
| 2573 | OOM | Zoom Airlines, Inc. | IATA: Z4, Callsign: Zoom, IATA Prefix: 389, Ceased Operations and filed for Bankruptcy protection on 28 August 2008. | CDN | AVCODES |
| 2574 | OOM | Zoom Airlines | IATA: Z4 | | ICAO1 |
| 2575 | OOND | ONE TWO SEVEN ONE SIX SIX | | | SCAC4 |
| 2576 | OONE | ONE ONE NINE EIGHT TWO THREE FOUR ( | | | SCAC4 |
| 2577 | OOOC | ONE ON ONE CARRIERS INC | | | SCAC3 |
| 2578 | OOOC | ONE ON ONE CARRIERS INC | | | SCAC4 |
| 2579 | OOOX | Osborne, Inc. | added 10/1997; eliminated 4/1998; 7/2000-7/2005 (not listed) | | NAKINA |
| 2580 | OOOX | Osborne, Inc. | added 10/97; eliminated 4/98; 7/2000 | | NRHS |
| 2581 | OOOX | OSBORNE INCORPORATED | | | PWRC |
| 2582 | OOOX | Osborne | | | RSRV |
| 2583 | OOPC | OWNER OPERATORS PETROLEUM CORP | | | SCAC3 |
| 2584 | OORX | Chartrand's Traffic Service | eliminated 10/1959 | | NAKINA |
| 2585 | OORX | Chartrand's Tank Car Service, Inc. | added 10/1958 | | NAKINA |
| 2586 | OORX | Chartrand's Tank Car Service | added 4/1958; eliminated 10/1958 | | NAKINA |
| 2587 | OORX | Chartrand's Tank Car Rentals | added 1/1958; eliminated 4/1958 | | NAKINA |
| 2588 | OORX | Chartrand's Tank Car Service | added 7/1953; 7/1954-10/1957; eliminated 1/1958 | | NAKINA |
| 2589 | OORX | Chartrand's Traffic Service | 1/1952; eliminated 7/1953 | | NAKINA |
| 2590 | OORX | Opelousas Oil Refinery | 4/1947 (not listed); 1/1950 | | NAKINA |
| 2591 | OORX | Chartrand's Traffic Service | Jan-52 | | NRHS |
| 2592 | OORX | Chartrand's Tank Car Service | 10/57; eliminated 1/58 | | NRHS |
| 2593 | OORX | Chartrand's Tank Car Rentals | added 1/58; eliminated 10/58 | | NRHS |
| 2594 | OORX | Chartrand's Tank Car Service, Inc. | added 10/58 | | NRHS |
| 2595 | OORX | Chartrand's Traffic Service | eliminated 10/59 | | NRHS |
| 2596 | OORX | CHARTRAND'S TANK CAR SERVICE (OPELOUSAS OIL REFINERY) | | | PWRC |
| 2597 | OOS | Ostravské opravny a strojírny, s.r.o. | | CZ | VKM |
| 2598 | OOSA | OOST ATLANTIC LIJN BV | | | SCAC4 |
| 2599 | OOSL | ONE ONE SIX FIVE THREE FOUR FIVE (1 | | | SCAC4 |
| 2600 | OOST | OOSTDYK, ENGLE, INC | | | SCAC3 |
