Johan Olaf Brochmann Nordhagen (16 March 1883 – 6 November 1925) was a Norwegian architect, engineer and artist.
He was born in Christiania as a son of artist Johan Nordhagen (1856–1956) and Christine Magdalene Brochmann Johansen (1858–1933). He was a brother of Rolf Nordhagen and through him an uncle of Per Jonas Nordhagen. In April 1909 in Ådal he married farmer's daughter Thora Hval (1887–1960).[1]
Nordhagen was educated as an engineer in Oslo and worked as an apprentice to architect Bredo Greve for several years before studying at the Royal Danish Academy of Art while also assisting Martin Nyrop with his designs for Copenhagen City Hall.
Nordhagen returned to Oslo in 1906 and accepted a number of smaller commissions. His breakthrough, however, came when he won the competition to build the Bergen Public Library in Jugendstil, for which he also won the Houen Prize.
Nordhagen designed a number of industrial structures, notably Såheim in Rjukan (with Thorvald Astrup); designed several churches; and completed considerable research around Norwegian traditional architecture, but he is probably most famous for the restoration designs of the Nidaros Cathedral. He took over project management of this work upon the death of Christian Christie and carried it through until he died. His tendency to reinterpret Gothic designs rather than seek a faithful reproduction of the original was controversial but was carried out.