You don't go through passport control in Norway unless you are departing or arriving from a non-Schengen country. So flying between Copenhagen and Oslo, for example, doesn't involve passport control.

Work permits in Norway used to be 3 valid for 3 years at a time, but they've had to shorten this to 2 years due to EU requirements. If you get permanent residency you also have to renew it every two years by making an appointment and visiting the police station. Luckily they don't charge you for renewals of permanent residency, just the initial application.