| OACX | The O.A. Cooper Co. | added 1/74; 1/75-1/80; eliminated 5/83 |
| Central Transportation Co., Inc. | eliminated 10/88 | |
| OALX | Oregon-American Lumber Corp. | 4/38-1/52; eliminated 1/53 |
| OAR | Old Augusta R.R. Co. | 1/94-7/2000 |
| OBCX | The Neatsfoot Oil Refineries Corp. | Apr-38 |
| OCCX | Occidental Chemical Co. | added 4/81; eliminated 1/83 |
| Occidental Chemical Agricultural Products, Inc. | added 1/83; eliminated 7/88 | |
| Occidental Chemical Corp. Agricultural Products Group | added 7/88; eliminated 4/89 | |
| Occidental Chemical Corp. Agricultural Products | added 4/89; eliminated 1/92 | |
| Occidental Chemical Corp. | added 1/92; 1/94-7/2000 | |
| OCE | Oregon, California & Eastern Ry. Co. | 1/75-1/80; eliminated 4/92; 1/94 |
| OCFX | Owens Corning Fiberglas | 1/94-7/2000 |
| OCPX | Occidental Chemical Corp., PVC Resins | added 1/88; eliminated 1/92 |
| Occidental Chemical Corp. | added 1/92; 1/94; eliminated 7/99 | |
| Oxy Vinyls L.P. | added 7/99; 7/2000 | |
| OCR | Oklahoma Central R.R. Co. | added 1/88; eliminated 7/90; 1/94-7/2000 |
| OCRR | Ottawa Central Ry. | added 4/2000; 7/2000 |
| OCRX | Oklahoma Central R.R. | Jul-00 |
| OCTL | Oil Creen & Titusville Lines | 1/94-7/2000 |
| OCTR | Octoraro Ry., Inc. | added 10/78; 1/80-1/94; eliminated 10/94; 7/2000 |
| OCTX | Octel America, Inc. | Jul-00 |
| OCVX | Gregory-Robinson-Speas, Inc. | Apr-38 |
| ODTX | Oil-Dri Corp. of America | added 10/96; added 7/99; 7/2000 |
| OD&W | Oneida & Western R.R. | 4/38-1/52 |
| OE | Oregon Electric Ry. | 1/52-7/65 |
| Oregon Electric Ry. Co. (Spokane, Portland & Seattle Ry. Co.) | Oct-66 | |
| Oregon Electric Ry. Co. | 1/75-1/80 | |
| OECX | Oldbury Electro-Chemical Co. | 4/38-10/57; eliminated 10/57 |
| OEDX | Ohio Edison Co. | 4/61-1/80; eliminated 5/83 |
| OELX | O.E. Linsin, Inc. | 4/38-4/70; eliminated 4/74 |
| OFCX | Ortner Freight Car Co. | added 10/67; 4/70-1/80; eliminated 7/87 |
| Residco | added 7/87; 1/94-7/2000 | |
| OFKX | Koch Tank Line, Inc. | 4/38-10/66; eliminated 1/67 |
| Witco Chemical Co., Inc. | added 1/67; eliminated 1/69 | |
| Witco Chemical Corp. | added 1/69; 4/70-1/75 | |
| OFOX | Residco | added 7/88; 1/94-7/2000 |
| OFSX | Ogden Financial Services Corp. | 1/94; eliminated 1/95 |
| Tucson Electric Power Co. | added 1/95; 7/2000 | |
| OGEE | Ogeechee Ry. Co. | added 10/89; eliminated 1/93; 1/94-7/2000 |
| OGEX | Oklahoma Gas and Electric Co. | added 7/76; 1/80-7/2000 |
| OGPX | Oklahoma Gasoline Plants, Inc. | Apr-38 |
| OGSX | Iowa Southern Utilities Co. | added 7/80; 1/94; eliminated 1/96 |
| IES Utilities, Inc. | added 1/96; eliminated 4/2000 | |
| Alliant Energy | added 4/2000; 7/2000 | |
| OHCR | Ohio Central R.R. Inc. | added 7/88; 1/94 |
| Ohio R.R. System | eliminated 10/92 | |
| The Columbus & Ohio River R.R. Co. | added 10/92 | |
| Ohio Central R.R. Inc. | 1/94-7/2000 | |
| OHCX | Old Hickory Chemical Co. | 4/38-4/61; eliminated 1/65 |
| Stauffer Chemical Co. -- Eastern Branch | added 4/65; 7/65 | |
| OHFX | Fluor Daniel Fernald | Jul-00 |
| OHGX | Otha H. Grimes | Apr-38 |
| Otha H. Grimes (Grimes Gasoline Co.) | Jan-52 | |
| OHIC | Ohi-Rail Corp. | added 10/82; eliminated 10/87; eliminated 1/91; 1/94-7/2000 |
| OHOX | U.S. Dept. of Energy Ohio Field | Jul-00 |
| OHPA | Ohio & Pennsylvania R.R. Co. | added 10/95; 7/2000 |
| OHPX | Merchants Despatch Transportation Corp. | 10/57-4/61 |
| OHRY | Owego and Harford Ry., Inc. | added 7/92; 1/94-7/2000 |
| OHTX | O&H Transport, Inc. | added 10/67; 4/70-1/80 |
| OIER | The Indiana & Ohio Rail System | eliminated 7/94 |
| OILX | Oil Tank Line Co. | 4/38-10/57; eliminated 10/60 |
| Transcontinental Transport, Inc. | added 10/60; 4/61 | |
| General Electric Railcar Services Corp. | 1/94; eliminated 10/94 | |
| OJGX | Oscar J. Glover | 1/94-7/2000 |
| OJTX | O.J. Tauber | 10/57-7/65; eliminated 10/65 |
| Tauber Transportation Co. | added 7/78; 1/80; eliminated 7/84; 1/94 | |
| OK | Oklahoma Ry. Co. | Apr-38 |
| OKAN | Okanagan Valley Ry. Co. | Jul-00 |
| OKCE | Okarche Central Ry., Inc. | added 10/82; eliminated 1/83 |
| OKCX | OKC Corp. | added 4/80; eliminated 10/81 |
| Louisiana Cement Co., Inc. | added 10/81 | |
| Lone Star Industries, Inc. | 1/94-7/2000; eliminated 1/2001; added 7/2001 | |
| OKEX | Okee, Inc. | Jan-75 |
| OKEX, Inc. | 1/80; eliminated 7/82 | |
| OKKT | Oklahoma, Kansas and Texas R.R. Co. | added 10/80; added 1/81; eliminated 4/82; added 1/83; eliminated 1/90 |
| Missouri Pacific System | added 1/90; 1/94; eliminated 4/97 | |
| Union Pacific R.R. Co. | added 4/97; 7/2000 | |
| OKLX | Oklahoma Tank Car Line | 10/66; eliminated 4/69 |
| OLB | Omaha, Lincoln and Beatrice Ry. Co. | added 10/74; 1/75-7/2000 |
| OLDX | Old Line Holding Co., Inc. | added 1/92; 1/94-7/2000 |
| OLNX | Olin Corp. | 1/80-7/2000 |
| OLO | Ontario L'Orignal Ry., Inc. | Jul-00 |
| OLYR | The Olympic R.R. Co. | added 7/89; 1/94-7/2000 |
| OLYX | Olympia Petroleum, Inc. | added 7/80 |
| OMAX | Omaha Public Power District | added 1/78; 1/80-7/2000 |
| OMCX | O-M Corp. | 1/75-1/80 |
| OMID | Ontario Midland R.R. Corp. | added 1/80; 1/80-7/2000 |
| OMLP | Ohio Midland Light & Power | 1/75-1/80 |
| OMLX | OmniTRAX Leasing, Ltd. | Jul-00 |
| OMRX | Oscar Meyer & Co. (cars no. 501-599, 801-899) (Mather Stock Car Co.) | Apr-38 |
| OMX | National Car Co. | Jan-52 |
| Fruit Growers Express Co. | 10/57-4/61 | |
| ONCT | Ontario Central R.R. Corp. | added 1/80; 1/80-7/2000 |
| ONER | Ontario Eastern R.R. Corp. | added 7/82; eliminated 4/86 |
| ONRY | Ogdensburg Bridge & Port Authority | Jan-75 |
| ONSX | Oglebay Norton Industrial Sands, Inc. | Jul-00 |
| ONT | Ontario Northland Ry. | 1/52-7/2000 |
| ONTA | Ontario Northland Ry. | added 1/81; 1/94-7/2000 |
| ONTX | On-Track Railcar Services, Inc. | added 4/88; 1/94-7/2000 |
| ONW | Oregon & Northwestern R.R. Co. | 1/75; added 4/79; 1/80; eliminated 4/89; eliminated 4/90 |
| ONYX | American Colloid Co. | 4/70; added 7/70; 1/75-1/80; eliminated 1/85 |
| OOOX | Osborne, Inc. | added 10/97; eliminated 4/98; 7/2000 |
| OORX | Chartrand's Traffic Service | Jan-52 |
| Chartrand's Tank Car Service | 10/57; eliminated 1/58 | |
| Chartrand's Tank Car Rentals | added 1/58; eliminated 10/58 | |
| Chartrand's Tank Car Service, Inc. | added 10/58 | |
| Chartrand's Traffic Service | eliminated 10/59 | |
| OPAX | Oxford Paper Co. | added 4/54 |
| Oxford Paper Co., Inc. | 10/57; eliminated 1/59 | |
| Oxford Paper Co. | added 1/59; eliminated 10/59 | |
| OPCX | Omaha Packing Co. (National Car Co.) | Apr-38 |
| The Olympic Portland Cement Co., Ltd. | added 4/54; 10/57; eliminated 7/59 | |
| Permanente Cement Co. | added 7/59; 4/61 | |
| General Electric Railcar Services Corp. | added 7/87; eliminated 1/88 | |
| OPE | Oregon Pacific and Eastern Ry. Co. | 1/75-1/80; eliminated 1/94; eliminated 4/95; 7/2000 |
| OPIX | OXY Petrochemicals Inc. | added 7/90 |
| OXY NGL, Inc. | eliminated 1/91 | |
| OXY USA Inc. | added 1/91 | |
| OXY Petrochemicals Inc. | eliminated 4/91 | |
| Occidental Chemical Corp., Petrochemicals Division | added 4/91; eliminated 1/92 | |
| Occidental Chemical Corp. | added 1/92; 1/94; eliminated 10/98 | |
| Equistar Chemicals, LP | added 10/98; 7/2000 | |
| OPPX | Omaha Public Power District | added 7/94; 7/2000 |
| OPR | Oregon Pacific R.R. Co. | Jul-00 |
| OPSX | Public Service Co. of Oklahoma | added 1/79; 1/80-7/2000 |
| OPT | Orange Port Terminal Ry. | Jul-00 |
| OPX | The Ohio Power Co. | 1/52-10/57; eliminated 10/58 |
| Ohio Power Co. | added 10/58; 4/61-7/2000 | |
| OR | Owasco River | 1/75-1/80 |
| ORA | Ormet R.R. Corp. | Jul-00 |
| ORCX | Oklahoma Refining Co. | Apr-38 |
| OREX | REA Express | added 10/70; eliminated 4/74 |
| Oregon Dept. of Agriculture | 1/94-7/2000 | |
| ORHX | Oscar R. Howard | Apr-38 |
| Howard Tank Line | 1/52-7/65; eliminated 7/66 | |
| ORMX | D.L. Orme | 1/52-10/66; eliminated 1/70 |
| OROX | U.S. Atomic Energy Commission | added 10/69; 4/70-1/75 |
| U.S. Atomic Energy Commission, Oak Ridge Operations Office | eliminated 4/75 | |
| United States Energy Research and Development Administration, Oak Ridge Operations Office | added 4/75; eliminated 1/78 | |
| U.S. Dept. of Energy, Oak Ridge Operations Office | added 1/78; 1/80-1/94; eliminated 7/95; 7/2000 | |
| ORR | Osage R.R. | 1/94; added 1/96; eliminated 10/96; 7/2000 |
| ORSX | Oxweld Railroad Service Co. | 1/52-4/61 |
| OSKX | Oil States Tank Car Corp. (Mid-Continent Petroleum Corp.) | Apr-38 |
| Mid-Continent Petroleum Corp. | Jan-52 | |
| D-X Sunray Oil Co. | 10/57-4/61 | |
| Sunray D-X Oil Co. | Jul-65 | |
| Oil States Tank Car Corp. (Sunray D-X Oil Co.) | Oct-66 | |
| Sunray D-X Oil Co. | eliminated 10/67 | |
| OSRR | Ohio Southern R.R., Inc. | added 10/87 |
| Ohio R.R. System | eliminated 10/92 | |
| The Columbus & Ohio River R.R. Co. | added 10/92 | |
| Ohio Southern R.R., Inc. | 1/94-7/2000 | |
| OSRX | Ontario Southland Ry. Inc. | Jul-00 |
| OT | Oregon Trunk Ry. | 4/38-7/65 |
| Oregon Trunk Ry. (Spokane, Portland & Seattle Ry. Co.) | Oct-66 | |
| Oregon Trunk Ry. | 1/75-1/80 | |
| OTCO | Owensville Terminal Co., Inc. | Jul-00 |
| OTDX | O-T-D Corp. | 4/61-1/75; eliminated 1/80 |
| Greenbrier Leasing Corp. | added 1/80; 1/80-7/2000 | |
| OTOX | Otto Milk Co. | Apr-38 |
| OTPX | Otter Tail Power Co. | 4/38-10/57; eliminated 10/59 |
| OTR | The Oakland Terminal Ry. | 1/75-7/2000 |
| OTRX | On-Track Railway Services Ltd. | Jul-00 |
| OTSX | On-Track Rail Services, LLC | Jul-00 |
| OTT | Ottumwa Terminal R.R. Co. | added 1/88; eliminated 7/90; 1/94-7/2000 |
| OTTX | Trailer Train Co. | 7/65; added 10/65; 10/66 |
| Trailer Train Co. (FMS flats specially equipped to provide for loading of agricultural equipment) | Oct-68 | |
| Trailer Train Co. | Apr-70 | |
| Trailer Train Co. (flat cars equipped with 64 chains, with snubbers, each secured to movable and retractable tie-down winches in four longitudinal channels, for transporting agricultural equipment) | 1/75-1/80 | |
| Trailer Train Co. | eliminated 1/92 | |
| TTX Co. | added 1/92 | |
| TTX Co. (flat cars equipped with 48 chains, with snubbers, each secured to movable and retractable tie-down winches in 4 longitudinal channels; for transporting agricultural equipment) | Jan-94 | |
| TTX Co. (flat cars equipped with 48 chains, with snubbers, each secured to movable and retractable tie-down winches in 4 longitudinal channels; for transporting agricultural equipment, equipped with end of car cush.) | Jul-00 | |
| OTVR | Otter Tail Valley R.R. Co., Inc. | added 4/87; 1/94; eliminated 10/99; 7/2000 |
| OUCH | Quachita R.R. | added 4/91; 1/94-7/2000 |
| OUCX | Orlando Utilities Commission | added 1/87; 1/94-7/2000 |
| OURD | The Ogden Union Ry. and Depot Co. | 1/75-7/2000 |
| OVEX | Ohio Valley Electric Co. | added 1/97; 7/2000 |
| OVOX | Oriental Vegetable Oils Co., U.S.A. (W.R. Grace & Co.) | Apr-38 |
| W.R. Grace & Co. | Jan-52 | |
| OW | New York, Ontario & Western Ry. | 4/38-1/52 |
| OWIX | Owens-Illinois Glass Co., Mill Division | Apr-61 |
| Old World Industries Ltd. | added 1/2001; eliminated 4/2001 | |
| Old World Transportation, Ltd. | added 4/2001 | |
| OWSX | Oil Well Supply Co. (1980) Ltd. | Jul-00 |
| OWTX | Oneida & Western Transportation | Jan-80 |
| Shamrock Coal Co., Inc. | added 1/83; eliminated 10/91 | |
| OZCX | Ozark-Mahoning Co. | 1/52-4/70 |
| OZKX | Ozark Car Corp. (Deep Rock Oil Corp.) | Apr-38 |
| Ozark Tank Car Co. | 1/52-4/61 | |
| Union Tank Car Co. | 7/65-4/70; eliminated 7/73 | |
| OZX | Ozone Co., Inc. | 4/38-10/66; eliminated 1/67 |