Biking Bergen – Oslo
This route is great for minimizing traffic, and at the same time getting great nature experiences from the bike seat. Here you get beautiful Hardanger, parts of the Norseman route up Eidfjord and over Hardangervidda, Vøringsfossen and beautiful stave churches before heading down towards Oslo along winding bike paths.
There are many good accommodations along the way, but camping is also great.
I recommend cycling from Bergen to Oslo since you can then finish the uphill climbs early, and I would have estimated about 5 days.