After
examen artium in 1879 Olsen studied medicine at the
Royal Frederick University of Kristiania, where he finished first part in 1882. He had developed a particular interest in mushrooms already as a younger boy. In 1883 he published a book on mushrooms,
Spiselig sop, which was reissued several times until 1924. From 1882 to 1885 he worked as an assistant at the institute for pathology and anatomy. He also resided with the mycologist
Oscar Brefeld in
Münster. He graduated from the medical studies in 1888, and worked some years as a physician in Kristiania. In 1893 he delivered his
doctoral thesis,
Om sop paa levende jordbund. In 1888 he discovered a method to produce condensed milk without sugar. He designed the items required for production, and is credited for being the founder of industrial milk conservation in Norway. The product "Viking Milk" was patented and introduced by him in 1891. The factory at Toten was taken over by the Swiss company
Henri Nestlé in 1897, while Sopp continued as technical and scientific manager until he retired in 1925. Sopp was the first to suggest classifying
fungi as belonging to neither
plantae nor
animalia, but to a third
kingdom, already in his thesis
Om sop paa levende jordbund from 1893, and in a subsequent article in the periodical
Nyt Tidsskrift. He was decorated as a Knight of the
Royal Norwegian Order of St. Olav in 1900. ==Botanist abbreviation==