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| ELR | Route | Section | Remarks |
|---|---|---|---|
| OAJ | Oakenshaw South Jn to Oakenshaw Jn | ||
| OBN1 | Oban Line | Crianlarich Jn to Crianlarich Lower Jn | |
| OBN2 | Oban Line | Crianlarich Lower Jn to Oban | |
| OCD | Ordsall Chord | Water Street Jn to Deal Street Jn | |
| OCL | Orchardhall Branch | Fouldubs Jn to NR boundary | Understood to be part closed |
| OGM | Ogmore Valley Branch | Tondu Jn to former Ogmore Jn | |
| OJS | Otford Jn to Sevenoaks Jn | ||
| OLD | Oldbury Branch | Understood to be disused | |
| OLL1 | Ordsall Lane Jn to Liverpool Road Old Station | ||
| OLW | Windsor Link | Ordsall Lane Jn to Windsor Bridge South Jn | |
| OME3 | Olive Mount Jn to Edge Lane Jn | ||
| OOC1 | Old Oak Common Depot | No. 1 Yard | |
| OOC2 | Old Oak Common Depot | No. 2 Yard | |
| OOS | Old Oak Sidings (Willesden) | ||
| ORG | Oldham Road Goods Branch | Miles Platting Jn to Oldham Road Goods | |
| OSC | Oakenshaw South Jn to Crofton East Jn | ||
| OVE | Ogmore Valley Extension Line | former Cefn Jn to Port Talbot Steelworks | |
| OWW | Oxford, Worcester, and Wolverhampton Line | Wolvercot Jn to Dudley | |
| OXC | Oxley Chord | Stafford Road Jn to Bushbury Oxley Jn | |
| OXD | Oxford Branch | Bletchley Flyover Jn to Oxford North Jn | |
| OXO | Oxcroft Opencast Branch | Oxcroft Jn to Oxcroft Opencast Site | Understood to be disused |
| OXW | Windermere Branch | Oxenholme Jn to Windermere |