| Railroad | Records | Links |
|---|
| Totals | 33,505 | 998 |
|---|
| Oberlin and La Grange Railway | 4 | |
| Ocilla and Irwinville Railroad | 5 | |
| Ocilla, Pinebloom and Valdosta Railway | 2 | |
| Ogden and Hot Springs Railway | 1 | |
| Ogdensburg and Lake Champlain Railroad | 34 | 2 |
| Ogdensburgh, Clayton and Rome Railroad | 15 | |
| Ogemaw and Northwestern Railway | 4 | |
| Ohio and Indiana Railroad | 10 | |
| Ohio and Mississippi Railroad | 118 | 2 |
| Ohio and Mississippi Railroad Eastern Division | 22 | 3 |
| Ohio and Mississippi Railroad Western Division | 11 | 1 |
| Ohio and Pennsylvania Railroad | 11 | |
| Ohio Central Railroad | 22 | |
| Ohio River Junction Railroad | 4 | |
| Ohio River Railroad | 5 | |
| Ohio Southern Railroad | 10 | |
| Ohio Valley Railway | 10 | 1 |
| Oil Creek and Allegheny River Railway | 6 | |
| Oil Creek Railroad | 21 | |
| Old Colony and Fall River Railroad | 21 | |
| Old Colony and Newport Railroad | 12 | |
| Old Colony Railroad | 20 | 3 |
| Old River and Kissatchie Railway | 4 | |
| Omaha Bridge and Terminal Railway | 5 | |
| Orange and Alexandria Railroad | 12 | 1 |
| Orange and Northwestern Railroad | 8 | |
| Orange Belt Railway | 8 | 1 |
| Oregon and California Railroad | 8 | |
| Oregon Pacific Railroad | 6 | |
| Oregon Railroad and Navigation Company | 28 | 3 |
| Oregon Railway and Navigation Co | 12 | |
| Oregon Short Line Railroad | 27 | 2 |
| Orford Mountain Railway | 6 | |
| Oswego and Rome Railroad | 14 | |
| Oswego and Syracuse Railroad | 29 | |
| Oswego and Troy Railroad | 14 | |
| Otis Railway | 4 | |
| Ottawa, Northern and Western Railway | 11 | |
| Owasco River Railroad | 7 | |
| Owensboro and Nashville Railroad | 6 | |
| Oxford and Henderson Railroad | | 1 |
| Ozark and Cherokee Central Railway | 5 | |
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