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Holtsmark published several translations from Old Norse into Norwegian: Heimskringla (with Didrik Arup Seip, two volumes, 1934); the Prose Edda (1950); Helgisaga Óláfs konungs Haraldssonar (1956); Sverris saga (1961), Hákonar saga Hákonarsonar (1964); and Orkneyinga saga (1970).
During the German occupation of Norway he was arrested by the Nazi authorities on 11 September 1941, together with Didrik Arup Seip and Anton Wilhelm Brøgger.