| 801 | ODRM | OLD DOMINION CHAPTER - NRHS INC | | | PWRC |
| 802 | ODRM | OLD DOMINION CHAPTER-NATIONAL RAILW | | | SCAC3 |
| 803 | ODRT | ODOM, R E TRUCKING INC | | | SCAC3 |
| 804 | ODRV | OVERDRIVE LOGISTICS INC | | | SCAC4 |
| 805 | ODRX | Ozark Refining Co. | 8/1920 | | NAKINA |
| 806 | ODRX | OZARK REFINING CO. | | | PWRC |
| 807 | ODS | State Air Company Odessa Airlines | IATA: 5K, Callsign: Odessa Air, IATA Prefix: 654, Founded in 1996. | UA | AVCODES |
| 808 | ODS | Odessa Airlines | | | ICAO1 |
| 809 | ODSC | ODE, SCOTT TRANSPORTATION | | | SCAC3 |
| 810 | ODSC | ONTARIO DESIGN MOULDINGS INC | | | SCAC4 |
| 811 | ODSI | ORION DISTRIBUTION SYSTEMS INC | | | SCAC3 |
| 812 | ODSQ | OHIO DELIVERY SERVICE | | | SCAC3 |
| 813 | ODSS | ODENTON SHELL SERVICE CENTER INC | | | SCAC3 |
| 814 | ODSU | METALL PROGRESS | | UA | BIC |
| 815 | ODSV | ORCA DELIVERY SERVICE | | | SCAC4 |
| 816 | ODSW | ORIENT DEFENDER SHIPPING COMPANY LI | | | SCAC4 |
| 817 | ODTI | OVERDRIVE TRANSPORTATION INCORPORAT | | | SCAC4 |
| 818 | ODTK | ODELL TRUCKING INC | | | SCAC3 |
| 819 | ODTK | OD TRUCKING LLC | | | SCAC4 |
| 820 | ODTL | ODANAH TRUCK LINE INC | | | SCAC4 |
| 821 | ODTM | ODOM TRUCKING COMPANY INC | | | SCAC4 |
| 822 | ODTP | ON-DEKK TRANSPORTATION LLC | | | SCAC4 |
| 823 | ODTT | ON DEMAND TRANSPORT INC | | | SCAC4 |
| 824 | ODTX | Oil-Dri Corp. of America | added 10/1996; added 7/1999; 7/2000-7/2005 | | NAKINA |
| 825 | ODTX | Oil-Dri Corp. of America | added 10/96; added 7/99; 7/2000 | | NRHS |
| 826 | ODTX | OIL-DRI CORPORATION OF AMERICA | | | PWRC |
| 827 | ODTX | Oil-Dri Corporation of America | | | RSRV |
| 828 | ODUM | ODUM TRUCKING COMPANY INC | | | SCAC3 |
| 829 | ODUM | ODUM FARMING COMPANY INC | | | SCAC4 |
| 830 | ODVR | OLD RIVER STEAMSHIP COMPANY | | | SCAC3 |
| 831 | ODVR | OLD RIVER STEAMSHIP COMPANY | | | SCAC4 |
| 832 | ODWC | OUTDOOR WORLD CORP | | | SCAC3 |
| 833 | ODWL | ODW LTS | | | SCAC4 |
| 834 | ODXS | ODOM TRANSPORT SERVICES LLC | | | SCAC4 |
| 835 | ODY | Odyssey International | IATA: OL, Callsign: Odyssey, Founded in June 1988. Ceased operations during April 1990. | CDN | AVCODES |
| 836 | ODY | Coval Air Ltd. | Callsign: Odyssey, | CDN | AVCODES |
| 837 | ODY | Odyssey International | | | ICAO1 |
| 838 | ODYC | ODYSEA CARRIERS S A | | | SCAC4 |
| 839 | ODYE | ODYSSEA SHIP LINE | | | SCAC3 |
| 840 | ODYK | ODYSSEY TANK LINES LLC | | | SCAC4 |
| 841 | ODYR | ODAY TRUCKING INC | | | SCAC4 |
| 842 | ODYS | ODYSSEY TOURS LTD | | | SCAC3 |
| 843 | ODYT | ODYSSEY TRANSPORTATION | | | SCAC3 |
| 844 | ODYT | ODYSSEY TRANSPORTATION | | | SCAC4 |
| 845 | ODZG | ORDONEZ, GERARDO | | | SCAC4 |
| 846 | OE | Cairo Air Transport Company (Cairo Aviation) | ICAO: CCE, Designator OE recalled by IATA | ET | AVCODES |
| 847 | OE | Air Omega Luftverkehrsgesellschaft mbH | ICAO: AOW, Callsign: Warlock, Designator OE recalled by IATA | D | AVCODES |
| 848 | OE | Express Air Services (Channel Islands) Ltd. | IATA designator changed to LS | GB | AVCODES |
| 849 | OE | Asia Overnight Express Corporation | ICAO: AOT, Callsign: Asia Overnight, IATA Prefix: 918, | RP | AVCODES |
| 850 | OE | Westair Commuter Airlines, Inc. / United Express | ICAO: SDU, Designator OE recalled by IATA | USA | AVCODES |
| 851 | OE | Out-Island Airways Ltd. | | GB | AVCODES |
| 852 | OE | Bedarfsflugunternehmen Dr. L. Polsterer | ICAO: OPV, Callsign: Polsterer, | A | AVCODES |
| 853 | OE | Lauda Air Luftfahrt AG | Callsign: Lauda Air, IATA Designator changed to OE | A | AVCODES |
| 854 | OE | Yeongnam Air Co. Ltd. | ICAO: ONA, Callsign: Yeongnam Air, Founded in 2008. Commenced operations on 25 July 2008. Ceased operations in December 2008. | ROK | AVCODES |
| 855 | OE | LaudaMotion GmbH | ICAO: LDM, Callsign: Expert, Ceased operations on 31 October 2020 as operations transferred to Lauda Europe. Prevoiusly Niki. Sold to LaudaMotion on 23 January 2018. | A | AVCODES |
| 856 | OE | Fly One Airlines S.r.l. | ICAO: FOE, Callsign: First Eagle, | RO | AVCODES |
| 857 | OE | Oregon Electric Ry. Co. (Spokane, Portland & Seattle Ry. Co.) | 10/1966 | | NAKINA |
| 858 | OE | Oregon Electric Ry. Co. | 1/1975-1/1980 | | NAKINA |
| 859 | OE | Oregon Electric Ry. | 1/1950-7/1965 | | NAKINA |
| 860 | OE | The Ohio Electric Ry. Co. | 8/1920 | | NAKINA |
| 861 | OE | Oregon Electric Ry. | 1/52-7/65 | | NRHS |
| 862 | OE | Oregon Electric Ry. Co. (Spokane, Portland & Seattle Ry. Co.) | Oct-66 | | NRHS |
| 863 | OE | Oregon Electric Ry. Co. | 1/75-1/80 | | NRHS |
| 864 | OE | *OHIO ELECTRIC RAILWAY CO. | | | PWRC |
| 865 | OE | *(BN) RETIRED 10-12-92 ATP [SPOKANE, PORTLAND & SEATTLE RWY. CO.; OREGON ELECTRIC RWY.(SPS)] | | | PWRC |
| 866 | OE | *ORION EXPRESS | | | PWRC |
| 867 | OEA | Orient Thai Airlines | IATA: OX, Callsign: Orient Thai, Designator OX* recalled by IATA | T | AVCODES |
| 868 | OEA | Orient Thai Airlines | IATA: OX, Callsign: Orient Thai, IATA designator changed to OX. Designator OX* reinstated by IATA | T | AVCODES |
| 869 | OEA | Orient Thai Airlines Company Limited | IATA: OX, Callsign: Orient Thai, IATA Prefix: 578, IATA designator changed from OX*. Designator OX* reinstated by IATA | T | AVCODES |
| 870 | OEA | Orient Thai Airlines | IATA: OX | | ICAO1 |
| 871 | OEBA | OYSTER BROTHERS AGWAY | | | SCAC3 |
| 872 | OEBA | OPERATION FORESTIER MARTIN DUBE &SO | | | SCAC4 |
| 873 | OEBB | Oensingen-Balsthal-Bahn | | CH | TRAFFIC |
| 874 | OEBB | Oensingen-Balsthal-Bahn | | CH | VKM |
| 875 | OEBN | OEBIN | | | SCAC4 |
| 876 | OEBU | OSTERREICHISCHE BUNDESBAHNEN | | | SCAC3 |
| 877 | OEBU | OSTERREICHISCHE | | | SCAC4 |
| 878 | OEC | Christophorus Flugrettungsverein | | | ICAO1 |
| 879 | OECC | OEC FREIGHT CHICAGO INC | | | SCAC4 |
| 880 | OECF | OEC FREIGHT WORLDWIDE CO LTD | | | SCAC4 |
| 881 | OECM | OEC (MTL) INC | | | SCAC4 |
| 882 | OECN | ONE FOUR EIGHT EIGHT ZERO SEVEN (14 | | | SCAC4 |
| 883 | OECO | OVERSEAS EXPORT CRATING CO INC | | | SCAC3 |
| 884 | OECP | OMEGA ENERGY CORP | | | SCAC4 |
| 885 | OECR | OPTRIM ENERGY CORPORATION | | | SCAC4 |
| 886 | OECX | Oldbury Electro-Chemical Co. | 7/1937-7/1954; 10/1957 (not listed); eliminated 10/1957 | | NAKINA |
| 887 | OECX | Oldbury Electro-Chemical Co. | 4/38-10/57; eliminated 10/57 | | NRHS |
| 888 | OECX | OLDBURY ELECTRO-CHEMICAL CO. | | | PWRC |
| 889 | OECY | OVERLAND ENTERPRISE CO | | | SCAC4 |
| 890 | OED | Orion Air Charter | Callsign: Orion Charter, | ZA | AVCODES |
| 891 | OED | Orion Air Charter | | | ICAO1 |
| 892 | OEDS | ORIENT EXPRESS DELIVERY SERVICE INC | | | SCAC3 |
| 893 | OEDX | Ohio Edison Co. | added 4/1958; 4/1961-1/1980; eliminated 5/1983 | | NAKINA |
| 894 | OEDX | Ohio Edison Co. | 4/61-1/80; eliminated 5/83 | | NRHS |
| 895 | OEDX | OHIO EDISON COMPANY | | | PWRC |
| 896 | OEDX | Ohio Edison | | | RSRV |
| 897 | OEDX | OHIO EDISON CO | | | SCAC3 |
| 898 | OEEI | OLD EPHRAIM EXPRESS INC | | | SCAC4 |
| 899 | OEF | Oberhessische Eisenbahnfreunde eV | Lollar | D | privat-bahn.de |
| 900 | OEF | Oberhessische Eisenbahnfreunde e.V. | | DE | VKM |
