Sandefjord Airport at Torp serves Vestfold and Telemark and is also used by travelers heading to Oslo. It is a separate airport well southwest of Oslo Airport at Gardermoen, so the ground journey should be included when comparing flights.
Train connections, with a transfer between the terminal and station, and airport coaches provide onward travel. Rental cars are useful for coastal towns and inland holidays. Check the full itinerary to your destination, especially for an early departure or a late arrival, rather than assuming Torp is in central Oslo.