A two-storey building on Bronson Avenue shutdown some neighbouring streets for about two hours.
Gladstone between Bronson at Somerset and Bronson and Gladstone have since been reopened.
Firefighters say they received the call at 1:20 Thursday afternoon.
The blaze began at the rear of 346 Bronson on the second floor.
No injuries have been reported, and the Ottawa's Fire Prevention Unit is investigating the cause.
One firefighter described the second storey as being "completely burnt out," and says the occupants will not be able to return home this Christmas.
