Olav Hagen was born and raised in Vingrom outside Lillehammer, and worked in the forest as a lumberjack, like many other Norwegian cross-country skiers in that era. His national breakthrough came at the 1947 Norwegian Championships when he placed fifth in the 17 km and sixth in the 30 km. At the 1948 Championships he won a silver medal in the 30 km race and qualified for the Olympics, where he was a member of Norway’s relay team and earned a bronze medal. In 1949 he placed sixth at Holmenkollen in the 18 km race. Hagen continued to participate into the 1950s, and won his only national title at the age of 36 in the relay with his club Vingrom IL in 1957, together with his brother Torgeir Hagen and Håkon Brusveen, later a gold medalist at Squaw Valley in 1960.