UK national who is in Norway as a posted worker
If you are a posted worker from a UK company, you must apply for a residence permit as a skilled worker to be allowed to work in Norway after 31 December 2020. To apply for a work permit as a skilled worker, you must pay a fee (this is an ordinary application and not an application according to the Brexit regulations).
If you are a posted worker from a company in another EU/EEA country, you can continue to work in accordance with EU/EEA regulations. If you will be in Norway for more than three months, you must apply for a residence card.