How to buy tickets?
Online ticket sale usually shuts down 3 hours before the concerts starts. Available tickets is then sold at the entrance. If you got the message that the concert was sold out, do make another try at the venue.
For the concerts in Den Norske Opera & Ballett, Oslo Konserthus, Universitetets aula and Edderkoppen, only numbered tickets are sold. All other venues have free seating or a combination of seating/standing.