Olsen got his debut for FC Nordsjælland on 23 July 2017 at the age of 17. He started on the bench, but replaced Emiliano Marcondes in the 95th minute in a 3–2 victory against Brøndby IF in the Danish Superliga.[3] Few days later, he played 72 minutes in a Danish Cup game against amateur side Vejgaard B, where he both scored and assisted a goal.