Fredrikstad is served by the Norwegian State Railway (http://www.nsb.no/) on the Oslo-Halden line. There are also numerous express busses between Oslo and the towns and cities in Østfold, including Fredrikstad.
There are two main airports serving eastern Norway, the larger one is Oslo Gardermoen OSL, which unfortunately is far north of Oslo while Fredrikstad is far south of Oslo. Nevertheless, there is a bus service line F11 between Fredrikstad and the airport taking slightly over two hours. Ryanair is flying to the both military and civil airport at Rygge near Moss 30 minutes from Fredrikstad. Also Norwegian and a selection of charter companies service this airport.
Some people find a good deal if they fly to Gothenburg instead, and take the train or drive from there. Gothenburg is even further from Fredrikstad than OSL is though the difference in time is less than an hour, but sometimes the tickets might be better there.
