| Abbreviation | Definition |
|---|---|
| OMID | Ontario Midland Railroad |
| OMID | Ontario Midland Railroad Corporation |
| OMLP | Ohio Midland Light and Power |
| OMLX | OmniTRAX Leasing |
| OMLX | OmniTRAX Leasing Limited |
| OMNX | Omnisource (Steel Dynamics) |
| OMRR | Otago Miniature Road and Rail |
| OMRX | Ozark Mountain Railcar |
| OMSX | Ohio Mulch (J. T. Leasing) |
| ONCT | Ontario Central Railroad |
| ONCT | Ontario Central Railroad Corporation |
| ONER | Ontario Eastern Railroad |
| ONEU | Ocean Network Express |
| ONEU | Ocean Network Express Holdings Ltd |
| ONSX | Oglebay Norton Industrial Sands |
| ONSX | Oglebay Norton Industrial Sands Incorporated |
| ONT | Ontario Northland Railway |
| ONTA | Ontario Northland Railway |
| ONTX | On-Track Railcar Services |
| ONTX | On-Track Railcar Services Incorporated |