Citizens of EEA member states and Switzerland are entitled to work in Norway without a work permit, provided that they register with the Norwegian authorities if their stay exceeds three months.
UK citizens do not need a work permit for short stays up to 90 days within a 180‑day period, but they must apply for a residence permit if they intend to work or stay long‑term in Norway following Brexit regulations.
Official guidance: https://www.udi.no/en/want-to-apply/residence-permit-eu-brexit/
Most other foreign nationals (“third-country nationals”) must hold a valid residence permit granting the right to work, which is usually tied to the type of employment they will engage in. These permits must generally be applied for before entering Norway, although some categories may apply from within the country depending on their legal residence status.
Work permits and employment‑based residence permits are administered by the Norwegian Directorate of Immigration (UDI), in cooperation with the Norwegian Labour and Welfare Administration (NAV) and the Norwegian police.
Once approved, the residence permit decision is sent electronically to the Norwegian embassy or consulate responsible for the applicant’s country of residence, where identity checks and document submission will occur before the visa for entry (where required) is issued.
In addition, any person working in Norway must be registered with the Norwegian National Registry and obtain a D-number or personal number and must be registered with Norwegian social security (Folketrygden).