In 1978, both the National Capital Commission and the City of Ottawa officially recognized the Notre Dame Cathedral Basilica of Ottawa as an historic monument. The cathedral is indeed the oldest surviving church in Ottawa. It stands on the site that was occupied by the first Catholic chapel open to both anglophones and francophones of Bytown.
The first plan dates from 1839. The work on the building's structure was conducted between 1841 and 1865, and the interior ornamentation, from 1876 to 1885. Missionnaries on temporary postings initiated the actual construction. It was only after the arrival of the Oblate Fathers in 1844 that the work proceeded on a regular basis.
In 1847, the poor mission church was prematurely transformed into a cathedral, the seat of the first Bishop of Bytown, Most Reverend Joseph-Eugène-Bruno Guigues, O.M.I. With the growth of the diocese under the second Bishop of Ottawa, Most Reverend Thomas Duhamel, and with the impetus of a visionary artist, Canon Georges Bouillon, the cathedral was finally completed in 1885. Dedicated to the Immaculate Conception in 1853, Notre Dame Cathedral was elevated to the status of basilica in 1879.