| Abbreviation | Definition |
|---|---|
| OTSX | On-Track Railway Services |
| OTT | Ottumwa Terminal Railroad |
| OTT | Ottumwa Terminal Railroad Company |
| OTTX | Trailer Train (TTX Corporation) |
| OTTX | TTX Company |
| OTVR | Otter Tail Valley Railroad (Genesee & Wyoming) |
| OTVR | Otter Tail Valley Railroad Company |
| OUCH | Ouachita Railroad |
| OUCX | Orlando Utilities Commission |
| OURD | Ogden Union Railway and Depot |
| OURD | Ogden Union Railway and Depot Company |
| OVEX | Ohio Valley Electric |
| OVEX | Ohio Valley Electric Company |
| OVNZ | Overnite Transportation |
| OVNZ | Overnite Transportation Company |
| OVO | Ottawa Valley Railway |
| OVR | Ohio Valley Railroad |
| OVR | Ottawa Valley RaiLink |
| OWEU | Owens Energy |
| OWIX | Old World Transportation |