Dozens of others were injured in the tragedy, which happened at a level crossing in the southwestern end of the Canadian capital, said fire department officials, adding that no injuries were reported on the train.
The front end of the bus was chopped off together with some passengers, while the Ottawa-Toronto train of VIA Railway Company was derailed some 100 to 200 feet down the rail.
Following the crash, local first responders rushed to the scene and took the injured, some in serious condition, to nearby hospitals.
A witness was quoted by local media as saying that the bus did not stop and appeared to drive through a lowered crossing barrier.
Sources said that all VIA trains on the Ottawa-Toronto route have been canceled.
Canadian Prime Minister Stephen Harper posted an online message to express his condolences to the families of the victims.
"Deeply saddened to hear about the bus-train collision in Ottawa this morning," Harper wrote. "Our thoughts & prayers are w/ the families of those involved."