The O-Train is a light rail rapid transit system in Ottawa, Ontario, Canada, operated by OC Transpo. The O-Train system has two lines, the electrically-operated Confederation Line and the diesel-operated Trillium Line. Since May 2020, Stage 2 construction has temporarily shut down Line 2, with replacement bus service being offered at all stations. When Line 2 reopens in 2024, it will extend southward to Limebank station and incorporate five newly constructed stations, as well as an additional line linking Line 2 to Ottawa International Airport which will replace the current bus service from route 97. By 2026, expansions along Line 1 and the construction of Line 3 stations in the west end are expected to be complete, bringing the system's length to 64.5 km (40.1 mi), four lines and 41 stations. The O-Train network is fully grade separated and does not have any level crossings with roads.

OC Transpo is the organisation that operates and plans public transport in the city of Ottawa. OC Transpo's system includes bus rapid transit, light rail, conventional bus routes, and door-to-door paratransit.