If it is an official visit you need an introduction letter from your employer and an invitation from the institution, company, university or organization in Norway. One (or both) of the letters must contain a guarantee/confirmation about who will cover the expenses in connection with the trip. Occasionally the Embassy may ask for a confirmation directly from the sponsor.
Documentation of valid travel and medical insurance (minimum coverage of EUR 30,000).
The insurance must be valid in all Schengen countries (not only Norway) and valid from the day you leave until, and including, the day you are back.
Norwegian Ministry of Foreign Affairs