List of reporting marks: O

Note: Marks ending in U are for container owners; marks ending in X are not common carriers; marks ending in Z are for trailers without flanged wheels. All other marks are of common-carrier railroads.

• OACX - Central Transportation, Inc.
• OAR - Old Augusta Railroad
• OCCU - ATS Container Leasing SA
• OCCX - Occidental Chemical Company
• OCE - Oregon, California and Eastern Railway
• OCEU - Oceanex Inc.
• OCFX - Owens Corning Fiberglass
• OCLU - Overseas Containers, Ltd.
• OCNU - Ocean Lines, Inc.
• OCPX - Occidental Chemical Company; Oxy Vinyls
• OCR - Oklahoma Central Railroad
• OCRR - Ottawa Central Railway
• OCRX - Oil Creek Refining Company
• OCSU - Scripps Institute of Oceanography
• OCTL - Oil Creek and Titusville Lines
• OCTR - Octoraro Railway
• OCTX - Octel America, Inc.
• ODTX - Oil-Dri Corporation of America
• OEDX - Ohio Edison Company
• OERX - Southern California Railway Museum
• OFCX - Ortner Freight Car Company
• OFOX - Residco
• OFSX - Tucson Electric Power Company
• OGEE - Ogeechee Railway
• OGEX - Oklahoma Gas and Electric Company
• OGSX - Iowa Southern Utilities Company
• OHCR - Ohio Central Railroad
• OHFX - Fluor Daniel Fernald
• OHIC - Ohi-Rail Corporation
• OHIO - Ohio Terminal Railway
• OHKU - Ohka American, Inc.
• OHOX - United States Department of Energy (Ohio Field)
• OHPA - Ohio and Pennsylvania Railroad
• OHRY - Owego and Harford Railway
• OILX - GE Rail Services
• OJGX - Oscar J. Glover
• OJTX - Tauber Transportation Company
• OKAN - Okanagan Valley Railway (OmniTRAX)
• OKCE - Okarche Central Railway
• OKCX - Lone Star Industries
• OKKT - Oklahoma, Kansas and Texas Railroad; Union Pacific Railroad
• OLAU - Occidental Chemical Corporation
• OLB - Omaha, Lincoln and Beatrice Railway
• OLDX - Old Line Holding Company, Inc.
• OLGX - Ineos Oligomers USA, LLC
• OLLU - Ocean Leasing, Ltd.
• OLNX - Olin Corporation
• OLO - Ontario L'Orignal Railway
• OLYR - Olympic Railroad
• OLYU - Olympic Container Corporation
• OLYR - Olympic Railroad
• OLYX - Olympia Petroleum, Inc.
• OLYZ - Showa Line, Ltd.
• OMAX - Omaha Public Power District
• OMID - Ontario Midland Railroad
• OMLP - Ohio Midland Light and Power
• OMLX - OmniTRAX Leasing, Ltd.
• OMNX - Omnisource; Steel Dynamics.
• OMRX - Ozark Mountain Railcar
• OMSX - Ohio Mulch; J. T. Leasing, Inc.
• ONCT - Ontario Central Railroad
• ONER - Ontario Eastern Railroad
• ONRR - Ohio and National Railway Company
• ONSX - Oglebay Norton Industrial Sands, Inc.
• ONT - Ontario Northland Railway
• ONTA - Ontario Northland Railway
• ONTX - On-Track Railcar Services, Inc.
• ONW - Oregon and Northwestern Railroad
• ONYX - American Colloid Company
• OOCZ - Orient Overseas Line, Inc.
• OOLU - OOCL
• OOOX - Osborne, Inc.
• OOUU - Orient Overseas Line, Inc.
• OPE - Oregon, Pacific and Eastern Railway
• OPIX - Equistar Chemicals
• OPPX - Omaha Public Power District
• OPR - Oregon Pacific Railroad
• OPSU - EMP
• OPSX - Public Service Company of Oklahoma
• OPT - Orange Port Terminal Railway
• OPX - Ohio Power Company
• OR - Owasco River Railway
• ORA - Ormet Railroad
• ORCU - BSL Transport
• OREX - Oregon Department of Agriculture
• OROX - United States Department of Energy (Oak Ridge Operations Office)
• ORR - Osage Railroad
• OSCZ - Intermodal Services, Inc.
• OSL - Oregon Short Line; Union Pacific Railroad
• OSRX - Ontario Southland Railway
• OSSU - Eurovos B.V.
• OTCO - Owensville Terminal Company
• OTCR - Oakdale Traction Corporation
• OTDX - Greenbrier Leasing Corporation
• OTPX - Otter Tail Power Company
• OTR - Oakland Terminal Railway
• OTRX - On-Track Railway Services, Ltd.
• OTSX - On-Track Railway Services, Ltd.
• OTT - Ottumwa Terminal Railroad
• OTTX - TTX Corporation
• OTVR - Otter Tail Valley Railroad (RailAmerica)
• OUCH - Ouachita Railroad
• OUCX - Orlando Utilities Commission
• OURD - Ogden Union Railway and Depot Company
• OVEX - Ohio Valley Electric Company
• OVNZ - Overnite Transportation Company
• OVO - Ottawa Valley Railway
• OVR - Ohio Valley Railroad
• OWIX - Old World Transportation, Ltd.
• OWLU - Ocean World Lines, Inc.
• OWSX - Oil Well Supply Company
• OWTX - Shamrock Coal Company
• OXYU - Eurotainer US, Inc.