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.

Light rail transit (LRT) is a form of passenger urban rail transit characterized by a combination of tram and rapid transit features. While its rolling stock is similar to that of a traditional tram, it operates at a higher capacity and speed and often on an exclusive right-of-way. In many cities, light rail transit systems more closely resemble, and are therefore indistinguishable from, traditional underground or at-grade subways and heavy-rail metros.