| 1 | O&G | Oakdale & Gulf Ry. Co. | 7/1923 | NAKINA | |
| 2 | O&G | OAKDALE & GULF RWY. CO. | PWRC | ||
| 3 | O&W | New York, Ontario & Western Ry. Co. | 8/1920 & 7/1930-7/1936 | NAKINA | |
| 4 | O&W | NEW YORK, ONTARIO & WESTERN RWY. CO. | PWRC | ||
| 5 | OAA | Oxley Aviation | ICAO1 | ||
| 6 | OAAS | OURADA LOGISTICS LLC | SCAC4 | ||
| 7 | OABC | O & B TRANSPORTATION INC | SCAC3 | ||
| 8 | OABI | OWENS & BRAY INC | SCAC3 | ||
| 9 | OABQ | ONE ABC ENTERPRISES (1 ABC ENTERPRI | SCAC3 | ||
| 10 | OAC | Oriental Airlines | ICAO1 | ||
| 11 | OACH | CENTRAL TRANSPORTATION CO. {O.A. COOPER CO.} | PWRC | ||
| 12 | OACH | OUACHITA TERMINALS INC | SCAC4 | ||
| 13 | OACN | OA CARGO & TRANSPORTATION | SCAC4 | ||
| 14 | OACU | HADJIKYRIAKOS INDUSTRIES LTD | SCAC4 | ||
| 15 | OACX | Central Transportation Co., Inc. | added 7/1983; eliminated 10/1988 | NAKINA | |
| 16 | OACX | The O.A. Cooper Co. | added 1/1974; 1/1975-1/1980; eliminated 5/1983 | NAKINA | |
| 17 | OACX | The O.A. Cooper Co. | added 1/74; 1/75-1/80; eliminated 5/83 | NRHS | |
| 18 | OACX | Central Transportation Co., Inc. | eliminated 10/88 | NRHS | |
| 19 | OACX | CENTRAL TRANSPORTATION CO. | PWRC | ||
| 20 | OACX | Central Transportation | RSRV | ||
| 21 | OAD | Orscom Tourist Installations Company | ICAO1 | ||
| 22 | OAD | NL | TRAFFIC | ||
| 23 | OADG | O & A DEHU GREENHOUSES LTD | SCAC4 | ||
| 24 | OADI | OHIO AUTO DELIVERY INC | SCAC4 | ||
| 25 | OAE | Omni Air International | IATA: OY | ICAO1 | |
| 26 | OAEA | ORANGE AVENUE EXPRESS INC | SCAC4 | ||
| 27 | OAEN | A-ONE (A-1) TRANSPORTATION | SCAC4 | ||
| 28 | OAFG | O/A FINDLAY GRAIN AND LIVESTOCK HAU | SCAC4 | ||
| 29 | OAFH | OCEANAIR FORWARDING INC | SCAC4 | ||
| 30 | OAFL | O&A TEX-PACK EXPRESS, INC | SCAC3 | ||
| 31 | OAFW | OASIS LOGISTICS LLC | SCAC4 | ||
| 32 | OAG | OLD AUGUSTA RAILROAD COMPANY | PWRC | ||
| 33 | OAHQ | OAHU TRANSPORT CO LTD | SCAC3 | ||
| 34 | OAIC | ORVILLES APPLIANCE INC | SCAC4 | ||
| 35 | OAIN | ODOI ASSOCIATES INC | SCAC4 | ||
| 36 | OAIT | O & I TRANSPORT INC | SCAC3 | ||
| 37 | OAIT | O & I TRANSPORT INC | SCAC4 | ||
| 38 | OAK | Oakway | USA | TRAFFIC | |
| 39 | OAKC | OAKMONT CARRIER CORP | SCAC3 | ||
| 40 | OAKD | OAKLAND TRANSPORTATION SERVICE INC | SCAC3 | ||
| 41 | OAKD | OAKLAND TRANSPORTATION INC | SCAC4 | ||
| 42 | OAKE | OAKES, CHARLES H | SCAC3 | ||
| 43 | OAKF | OAKCREST FUEL INC | SCAC4 | ||
| 44 | OAKG | OAK GROVE TRUCKING COMPANY | SCAC3 | ||
| 45 | OAKH | OAK HARBOR FREIGHT LINES, INC | SCAC3 | ||
| 46 | OAKH | OAK HARBOR FREIGHT LINES INC | SCAC4 | ||
| 47 | OAKI | OAKLAND VAN & STORAGE INC | SCAC3 | ||
| 48 | OAKK | OAKLEY TANK LINES INC | SCAC4 | ||
| 49 | OAKL | OAKS LUMBER&HARDWARE, INC | SCAC3 | ||
| 50 | OAKL | OAKLEY DIE & MOLD CO INC | SCAC4 | ||
| 51 | OAKP | OAKLAND PORT SERVICES CORP | SCAC4 | ||
| 52 | OAKQ | OAK PARK TRANSFER | SCAC3 | ||
| 53 | OAKT | OAKS TRAILER TOWING | SCAC3 | ||
| 54 | OAKT | Oakville Transit | CDN | TRAFFIC | |
| 55 | OAKU | OAKSHOTT LIMITED | SCAC3 | ||
| 56 | OAKV | OAKS VAN LINES | SCAC3 | ||
| 57 | OAKV | OAKHAVEN RANCH INC | SCAC4 | ||
| 58 | OAKW | OAKWOOD TRUCKING CORP | SCAC3 | ||
| 59 | OAKY | OAKRIDGE TRUCKING | SCAC4 | ||
| 60 | OAL | Olympic Airlines | IATA: OA | ICAO1 | |
| 61 | OALU | OCEAN AFRICA CONTAINER LINE PTY LTD | SCAC4 | ||
| 62 | OALX | Oregon-American Lumber Corp. | 7/1937-1/1952; eliminated 1/1953 | NAKINA | |
| 63 | OALX | Oregon-American Lumber Corp. | 4/38-1/52; eliminated 1/53 | NRHS | |
| 64 | OALX | OREGON-AMERICAN LUMBER CORP. | PWRC | ||
| 65 | OAMX | Omaha Alfalfa Milling Co. | 8/1920-7/1923 | NAKINA | |
| 66 | OAMX | OMAHA ALFALFA MILLING CO. | PWRC | ||
| 67 | OANI | OCEANICA PRODUCTS | SCAC4 | ||
| 68 | OAO | Arkhangelsk 2 Aviation Division | ICAO1 | ||
| 69 | OAP | COAPA AIR | ICAO1 | ||
| 70 | OAQC | OMEGA-3 AQUACULTURE | SCAC4 | ||
| 71 | OAR | Old Augusta R.R. Co. | added 7/1983; 1/1994-7/2005; eliminated 7/2006 | NAKINA | |
| 72 | OAR | Old Augusta R.R. LLC | added 7/2006 | NAKINA | |
| 73 | OAR | Old Augusta R.R. Co. | 1/94-7/2000 | NRHS | |
| 74 | OAR | OLD AUGUSTA RAILROAD COMPANY | PWRC | ||
| 75 | OAR | Old Augusta Railroad | RSRV | ||
| 76 | OAR | OLD AUGUSTA RAILROAD LLC | SCAC4 | ||
| 77 | OARD | ORCHARD PARK GROWERS LTD | SCAC4 | ||
| 78 | OARK | OLD ARK TRUCKING LLC | SCAC4 | ||
| 79 | OASB | ORIENT ALLIANCE SHIPPING COMPANY LI | SCAC4 | ||
| 80 | OASC | OASIS CARGO | SCAC3 | ||
| 81 | OASC | OCEAN ANGEL SHIPPING CO | SCAC4 | ||
| 82 | OASD | OASIS DENTAL GROUP INC | SCAC4 | ||
| 83 | OASE | OASIS ENTERPRISE INC | SCAC3 | ||
| 84 | OASI | OASIS COMPANY INC | SCAC3 | ||
| 85 | OASI | OASIS ENE | SCAC4 | ||
| 86 | OASK | OCEGUERA AND SONS TRUCKING | SCAC4 | ||
| 87 | OASL | OASIS ALIGNMENT SERVICES LTD | SCAC4 | ||
| 88 | OASS | OASIS LINES INC | SCAC3 | ||
| 89 | OATC | OLMSTEAD, ARCHIE TRUCK CO | SCAC3 | ||
| 90 | OATD | OATES, DAVID | SCAC3 | ||
| 91 | OATE | OATES BROS, INC | SCAC3 | ||
| 92 | OATE | OATES BROTHERS TRUCKING INC | SCAC4 | ||
| 93 | OATG | OLVERA TRUCKING | SCAC4 | ||
| 94 | OATI | OMEGA TRANSPORTATION INC | SCAC3 | ||
| 95 | OATJ | ORIENTAL AIR TRANSPORT SERVICE INC | SCAC4 | ||
| 96 | OATN | OCEAN AIR TRANSPORTATION INC | SCAC3 | ||
| 97 | OATP | OAT ENTERPRISES INC | SCAC3 | ||
| 98 | OATR | OCEANSIDE AUTO TRANS INC | SCAC3 | ||
| 99 | OAV | Omni - Aviacao e Tecnologia | ICAO1 | ||
| 100 | OAW | Helvetic Airways | IATA: 2L | ICAO1 |