Going around the same time.
It is a hard question since we don't really know your interests and what you want to see also it seems most tourists do the NIN routes and don't venture to some lesser publicized areas.
It is also a large country, not wide but tall and many times you cannot drive what would be a straight route to get places and need ferries as well.
For this reason I suggest visiting a website like VisitNorway.com looking at the different options ; that is an excellent website. Spend a good amount of time on the site, selecting a handful of spots that really catch your eye and then pinning them on a map to see how far away different places are, what is feasible.
After you determine a main spot to go, look for smaller sites not too far off and you can make a day out of it.
I have only been to Oslo before and while a nice, pretty and very clean city it didn't have much that wowed me as a tourist, since not sure it is a must on a Norway itinerary personally. If I want a city vacation in Europe it would always be better to go to Amsterdam, Paris, Rome, Florence, London, etc...
I am sure opinions will differ though.
Figured I would share how I came about my plans which may help you.
Originally I had planned on flying into and out of Bergen ; while I found quite a few places to drive to from Bergen the #1 spot for us was Geiranger and it's fjord ; in my top 5 were also nearby Romsadalen Ridge and Trollstigen. I realized all 3 of these places were too far a drive from Bergen though so ended up changing my plans to fly into and out of Alesund which is much closer to all 3 areas.
It of course meant Bergen and all of the cool waterfall and hiking spots around Bergen as well as Stave Churches and other fjords had to now go as well.
So my plan is now limited to a fairly small area where there are a couple of fjords, Geiranger being the main one and the mountains I was looking for. For our 5 nights it made sense and the smaller city of Aselund will be our only taste of the city.
Not saying to change your flight or lodging just to identify how far you can travel and what interests you the most:
Fjords, hiking, waterfalls, city walks, walk on a glacier, boat rides, train rides, stave churches, etc...