Flight
Oslo airport is the best airport to arrive at and closest airport to Fredrikstad. From Oslo airport it is a 1.5 hours drive to Fredrikstad.
Train
The train leaves from the airport and is a 2 hour trip usually with a change in South Oslo. Alternatively, the bus is also available from the Oslo airport to the Fredrikstad Bus terminal which you can buy tickets and view timetable. https://www.nettbuss.no
Hotel
Hotel
Hotel
House Or Rental
There are plenty of good Airbnb options in Fredrikstad and Old town Fredrikstad under $100 per night if you would like an alternative to a hotel.