hi Petter,
Oslofjord is very good for trout and salmon fishing.
The fish goes on top in winter too, intermeddiate is perfect.. I catch fish in Oslofjord from 3 to 4 kilos fish about 2 weeks ago,-5 degrees, they are surfacing from the top..and most of my seatrouts these winter was taken when they surface on top ( not a dry fly, but a fly that sinks slowly)..If you go on about a meter deep you have more chance on winter. . I even catch trout, torske and lyr in -11 ( trout and lyr surfacing on top, torske is by sightfishing). There is more chances in shallow water than in deep water on a fly, and it is easier to find a fish.theres more food on shallow than in deep for the fish. If theres one, theres possible many friends more, or a mother and father
Orange and pink flies is great.Retrieve about 1.5 seconds interval works best for me..Sunrise, middle of the day and sunset..
I hope this helps.. Good luck to you and wish you best in fishing..