Johan Olaf Brochmann Nordhagen (16 March 1883 – 6 November 1925) was a Norwegian architect, engineer and artist. He is most commonly associated with his restoration designs for Nidaros Cathedral in Trondheim, Norway.
Olaf Nordhagen 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 Thora Hval (1887–1960).
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 Fund prize (Houens fonds premie).
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. Dating from 1913, he was also a professor at the Norwegian Institute of Technology in Trondheim. 
Olaf Nordhagen 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 his own death in 1925. His tendency to reinterpret Gothic designs rather than seek a faithful reproduction of the original was controversial at the time but was carried out. 
- Arkitekter: Johan Olaf Brochmann Nordhagen (Norske arkitekter og arkitekter på norsk grunn)
- Nidaros Cathedral (memim.com)
- Lunde, Øivind. "Olaf Nordhagen". In Helle, Knut. Norsk biografisk leksikon (in Norwegian). Oslo: Kunnskapsforlaget. Retrieved 26 December 2010.
- Olaf Nordhagen, Ingeniør, arkitekt og kunstner (Høgskolen i Oslo og Akershus)
- Olaf Nordhagen, arkitekt (Norsk kunstnerleksikon. Forfatter,Dag Myklebust)
- Johan Olaf Brochmann Nordhagen (Den Store Danske)
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