I want to fly a drone or a model airplane within 5 kilometers of an airport, whom do I contact?

You have to contact Air Traffic Management at the airport in question. Airports may have different procedures for how to apply for a permit. Most use an app calles Ninox Drone. You can also call the AIS/NOTAM office at Gardermoen (AIS = Aeronautical Information Service, NOTAM = Notice to airmen). You’ll find the contact information at Avinor Internet Pilot Planning Centre (IPPC), see link below.