| 601 | OCS | Ocean Sky Aviation Ltd | Callsign: Ocean Sky, Ceased operations. | GB | AVCODES |
| 602 | OCS | CoyotAir, S.A. | Callsign: Yoco, | E | AVCODES |
| 603 | OCS | Ocean Sky (UK) | | | ICAO1 |
| 604 | OCSA | OCEANICA, S. A. | | | SCAC3 |
| 605 | OCSA | ORIENT CHAMPION SHIPPING COMPANY LI | | | SCAC4 |
| 606 | OCSC | OCEANIC CREST SHIPPING COMPANY LIMI | | | SCAC3 |
| 607 | OCSD | OCEANSIDE SHIPPING ENE | | | SCAC4 |
| 608 | OCSF | OCEANSIDE FISHERIES INC | | | SCAC4 |
| 609 | OCSG | OCS LOGISTICS | | | SCAC4 |
| 610 | OCSH | OCEANIC SHIPPING COMPANY OF GEORGIA | | | SCAC3 |
| 611 | OCSH | OCEAN COMMAND SHIPPING COMPANY LIMI | | | SCAC4 |
| 612 | OCSI | OCS TRANSPORTS INC | | | SCAC3 |
| 613 | OCSI | OCEAN AGENCIES INC | | | SCAC4 |
| 614 | OCSJ | ORIENT CENTAUR SHIPPING COMPANY LIM | | | SCAC4 |
| 615 | OCSK | OCEAN CRYSTAL SHIP AND SEATRADE INC | | | SCAC4 |
| 616 | OCSL | OCEANFOCUS SHIPPING LTD OWNERS OF M | | | SCAC3 |
| 617 | OCSL | OCEANFOCUS SHIPPING LTD OWNERS OF M | | | SCAC4 |
| 618 | OCSM | AMERICAN PRESIDENT INTERMODAL COMPANY | | | PWRC |
| 619 | OCSN | OCEAN CREST SHIPPING COMPANY LIMITE | | | SCAC4 |
| 620 | OCSO | OCEAN COMPASS SHIPPING COMPANY LIMI | | | SCAC4 |
| 621 | OCSP | OCS TRANSPORT SERVICES INC | | | SCAC4 |
| 622 | OCSR | OCEAN CURRENT SHIPPING COMPANY LIMI | | | SCAC4 |
| 623 | OCSS | OCEANIC STEAMSHIP LINES | | | PWRC |
| 624 | OCSS | OCEANIC STEAMSHIP COMPANY, THE | | | SCAC3 |
| 625 | OCSS | OCEANIC STEAMSHIP COMPANY, THE | | | SCAC4 |
| 626 | OCSU | SCRIPPS INSTITUTE OF OCEANOGRAPHY | | | PWRC |
| 627 | OCSU | Scripps Institute of Oceanography | | | RSRV |
| 628 | OCSU | OVERSEAS CONTAINER LTD | | | SCAC3 |
| 629 | OCSU | OVERSEAS CONTAINER LTD | | | SCAC4 |
| 630 | OCSV | OCEAN CHARIOT SHIPPING COMPANY LIMI | | | SCAC4 |
| 631 | OCSX | Omaha Cold Storage Co. (North Western Refrigerator Line Co.) | 7/1935 | | NAKINA |
| 632 | OCSX | Omaha Cold Storage Co. | 7/1932 | | NAKINA |
| 633 | OCSX | OMAHA COLD STORAGE CO. | | | PWRC |
| 634 | OCSY | ORIENT CAVALIER SHIPPING COMPANY LI | | | SCAC4 |
| 635 | OCT | Occidental Airlines (Liberia), Inc. | Callsign: Occidental, | LB | AVCODES |
| 636 | OCT | Octavia Air Ltd. | Callsign: Octavia, | GB | AVCODES |
| 637 | OCT | OCTORARO RAILWAY INC | | | PWRC |
| 638 | OCTA | OVERSEAS COAL TRANSPORT INC. | | | SCAC3 |
| 639 | OCTA | Orange County Transportation Authority | | US-CA | TRAFFIC |
| 640 | OCTB | O/C TRANSPORTATION | | | SCAC4 |
| 641 | OCTD | OCEAN TRANSPORT LTD | | | SCAC4 |
| 642 | OCTD | Orange County Transit District | | USA | TRAFFIC |
| 643 | OCTG | OCEAN CITY TRUCKING INC | | | SCAC3 |
| 644 | OCTG | O C TRUCKING | | | SCAC4 |
| 645 | OCTH | OCEANIC TRUCKING CO OF TEXAS INC | | | SCAC3 |
| 646 | OCTI | OCEAN TRUCKING | | | SCAC3 |
| 647 | OCTI | OCTAVE INTERNATIONAL HOLDINGS LTD | | | SCAC4 |
| 648 | OCTJ | OCEAN TRANSPORT LLC | | | SCAC4 |
| 649 | OCTL | Oil Creen & Titusville Lines | 1/1994-7/2005 | | NAKINA |
| 650 | OCTL | Oil Creen & Titusville Lines | 1/94-7/2000 | | NRHS |
| 651 | OCTL | OIL CREEK & TITUSVILLE LINES | | | PWRC |
| 652 | OCTL | Oil Creek and Titusville Lines | | | RSRV |
| 653 | OCTL | OIL CREEK & TITUSVILLE LINES | | | SCAC3 |
| 654 | OCTL | OIL CREEK & TITUSVILLE LINES | | | SCAC4 |
| 655 | OCTN | OCEANIC TRANSPORTATION INC | | | SCAC4 |
| 656 | OCTO | O AND C TRANSPORT LLP | | | SCAC4 |
| 657 | OCTQ | OCONNOR TRANSPORT INC | | | SCAC3 |
| 658 | OCTR | Morristown & Erie Ry., Inc., d/b/a Octoraro R.R. | eliminated 4/2007 | | NAKINA |
| 659 | OCTR | Morristown & Erie Ry., Inc., d/b/a Ontario R.R. | added 10/2003; 7/2005 | | NAKINA |
| 660 | OCTR | Octoraro Ry., Inc. | added 10/1978; 1/1980, eliminated 10/1985; 1/1994; eliminated 10/1994; 7/2000 | | NAKINA |
| 661 | OCTR | Octoraro Ry., Inc. | added 10/78; 1/80-1/94; eliminated 10/94; 7/2000 | | NRHS |
| 662 | OCTR | MORRISTOWN & ERIE RWY. CO. d/b/a OCTORARO RAILROAD | | | PWRC |
| 663 | OCTR | Octoraro Railway | | | RSRV |
| 664 | OCTR | OCTORARO RAILWAY INC | | | SCAC3 |
| 665 | OCTR | OCTORARO RAILROAD INC | | | SCAC4 |
| 666 | OCTR | OC Transpo | City: Ottawa | CDN | TRAFFIC |
| 667 | OCTU | OCEAN CONTAINERS TRADING S.L. | | E | BIC |
| 668 | OCTW | ORANGE COUNTY TOWING SERVICE INC | | | SCAC3 |
| 669 | OCTX | Octel America, Inc. | 7/2000-7/2005 (not listed) | | NAKINA |
| 670 | OCTX | Octel America, Inc. | Jul-00 | | NRHS |
| 671 | OCTX | OCTEL AMERICA INCORPORATED | | | PWRC |
| 672 | OCTX | Octel America | | | RSRV |
| 673 | OCTX | OCTEL AMERICA, INC. | | | SCAC4 |
| 674 | OCTY | ONTARIO CONTAINER TRANSPORT INC | | | SCAC4 |
| 675 | OCUR | OLMSTEAD CURRENCY | | | SCAC4 |
| 676 | OCVF | OCEANVIEW FUEL | | | SCAC4 |
| 677 | OCVT | OCV TRANSPORT CORP | | | SCAC4 |
| 678 | OCVU | O.C.C. OVERBEEK CONTAINER CONTROL BV | | NL | BIC |
| 679 | OCVU | O C C OVERBEEK CONTAINER CONTROL | | | SCAC4 |
| 680 | OCVX | Gregory-Robinson-Speas, Inc. | 7/1935-1/1950 | | NAKINA |
| 681 | OCVX | Ozark Cider & Vinegar Co. | 8/1920-7/1923 | | NAKINA |
| 682 | OCVX | Gregory-Robinson-Speas, Inc. | Apr-38 | | NRHS |
| 683 | OCVX | *OZARK CIDER & VINEGAR CO. | | | PWRC |
| 684 | OCVX | *GREGORY-ROBINSON-SPEAS, INC. | | | PWRC |
| 685 | OCW | Ocean Airways | IATA: OG, Callsign: Ocean Airways, Designator OG recalled by IATA | F | AVCODES |
| 686 | OCW | Ocean Airways | Callsign: Ocean Airways, Designator OG recalled by IATA | F | AVCODES |
| 687 | OCW | OCEAN CITY WESTERN RAILROAD | | | PWRC |
| 688 | OCWH | OX CARTAGE & | | | SCAC4 |
| 689 | OCWS | OCEAN WORLD SHIPPING LTD | | | SCAC4 |
| 690 | OCXI | OCEANEX INC | | | SCAC3 |
| 691 | OCXI | OCEANEX INC | | | SCAC4 |
| 692 | OCXL | OCEANLINX INTERNATIONAL LLC | | | SCAC4 |
| 693 | OCXP | OCEAN EXPRESS | | | SCAC3 |
| 694 | OCXR | OCEAN CARGO EXPRESS | | | SCAC4 |
| 695 | OCXS | OCONCA SHIPPING (LAX) INC | | | SCAC4 |
| 696 | OCYC | ORANGE COAST YELLOW CAB | | | SCAC3 |
| 697 | OCYS | OTD CYCLESPORTS | | | SCAC4 |
| 698 | OD | Zuliana de Aviacion .C.A. | ICAO: ULA, Callsign: Zuliana, Designator OD recalled by IATA | YV | AVCODES |
| 699 | OD | Natalco Airlines Limited | Designator OD recalled by IATA | ZRE | AVCODES |
| 700 | OD | Transportes Aereos Mercantiles Panamericanos S.A., (TAMPA) | | CO | AVCODES |
