He was born in Oslo as a son of Lieutenant Colonel Toralf Øyen (1878–1963) and Elia Haugros (1876–1951). He took his examen artium at Riis in 1934. He enrolled in medicine studies in 1936, but his studies were interrupted by World War II. He participated in the Battle of Midtskogen on 10 April 1940, one day after Germany invaded Norway. He had finished basic officer training in 1936, and holding the rank of Second Lieutenant in 1940 he continued to participate in battles north of Midtskogen, in Østerdalen. Norway soon lost the battle phase of the Norwegian–German conflict, though, and entered a phase of occupation and secret resistance work.