University of Oslo.
He was a professor at the University of Oslo, and his speciality was the Middle Ages. Schreiner finished his secondary education in 1921, and enrolled at the University of Oslo where he majored in history in 1927. He lectured at the University of Oslo from 1939, and was appointed professor in 1946.
Following the outbreak of World World War II he was arrested first in February 1942, being incarcerated at Bredtveit concentration camp until March.
He was later arrested in October 1943 along with other University faculty. He was incarcerated in Bredtveit concentration camp from October to November 1943, Berg concentration camp from November to December 1943 and Grini concentration camp from December 1943 to December 1944.
In 1946 and 1947 he co-edited the two-volume book Griniboken along with August Lange, with contributions from several of the prisoner at Grini. He chaired the society Den norske historiske forening from 1947 to 1955, and edited the society"s journal Historisk tidsskrift during the same period.
He was main editor of the five volume Trondheim bys historie (1956–1973), and Vårt folks historie (1961–1964).
From 1947 to 1955 he chaired the Norwegian Historical Association.