O
- Oakland and Ottawa Railroad
- Ogdensburg and Lake Champlain Railroad
- Ogdensburg Railroad
- Omemee, Bobcaygeon and North Peterborough Junction Railway
- Ontario, Belmont and Northern Railway
- Ontario Car Ferry Company, (Limited)
- Ontario and Ottawa Railway
- Ontario and Rainy River Railway
- Ontario Sault Ste. Marie Railway
- Ontario, Simcoe and Huron Railroad Union Company
- Oshawa Railway and Navigation Company
- Ottawa, Arnprior and Parry Sound Railway
- Ottawa, Arnprior and Renfrew Railway
- Ottawa and Parry Sound Railway
- Ottawa, Rideau Valley and Brockville Railway
- Ottawa Terminals Railway
- Oxford and Port Austin Railroad
Read more about this topic: List Of Canadian National Railways Companies