If you are a member of the Norwegian National Insurance scheme, you must pay National Insurance contributions. Your National Insurance contributions is deducted from your pay as part of the tax deducted.
If you are insured in an EU/EEA country or in another country with which Norway has signed a national insurance agreement, you do not have to pay National Insurance contributions in Norway. You must document this by presenting form A1 or other documentation issued by the national insurance authorities in your home country. Your tax withholding can then be reduced.
More information about the Norwegian National Insurance scheme is available at nav.no.