If you want to see fjords, don't go to Oslo. It's basically wrong side of the country. The most famous fjords is on the western coast, 6-7 hours by car/train away from Oslo. You can get pretty cheap airfares between Oslo and Bergen, so that might be cheaper than flying directly to Bergen - but KLM operates AMS-BGO as well.

You have quite a few famous hikes on the west coast, for instance Trolltunga.

Remember that in Norway hiking and camping is generally allowed outside farmed areas, and you can stay at the same place for up to three days.

ut.no has a lot of suggestions. Yes, it's in norwegian, but combine it with a map, and google translate, and you should be able to get some suggestions.

May I suggest Innderdalen and Mardalsfossen? Both are one-day visits from the parkinglot and has some amazing nature for you to camp in!

I will recomand Rallarvegen, its i biketrip on 52km and start 1222 meters above sealevel and ends at the sea. They have bikerental so you can get the bike on the top and delivery it at the bottom.
You can choise to start at Finse or Haugastøl. Finse have the railroad station in Norway who is highest over the sea.
Oslo to Finse with train take 4,5 hour

https://www.haugastol.no/en/rallarvegen
https://www.visitflam.com/en/

Hardangervidda is nice if you want a 4-5 days hike. Easy terrain to walk and lots of routes of different lengths between cabins you can stay in. If you start out on the east side it's not too far by train from Oslo, depending on how far you want to walk, and you end your hike by the fjords on the west side. Trolltunga was mentioned, then you end up in Odda, in Sørfjorden (an arm of Hardangerfjorden). You can also go to Eidfjord, and see Vøringsfossen on the way down to Hardangerfjorden. If you go Rallarveien you arrive in Flåm, in Aurlandsfjorden (part of Sognefjorden).

If you're driving all the way from the Netherlands to Norway, best option
for seeing fjords is to catch the ferry from Hirtshals in Denmark to Stavanger or
Bergen. If you decide to get off in Stavanger the drive from there to Bergen
has many good options for hiking. I suggest a drive through Hardanger, where
Jondal is a nice starting point for walking up into the mountains - here you also
have the opportunity to check out the fantastic glacier Folgefonna.

But - and I don't mean to be condescending - if this is your first trip to Norway
you should plan your trip thoroughly and well in advance. For hiking you
should choose your route with care, checking, among other things, what
possibilities you have in case of unforeseen emergencies. Also note that
the Norwegian summers are not necessarily warm, especially in higher
altitudes it can get quite cold even in July. It is wise to be well acquainted
with the area you plan to visit.

You should perhaps get in touch with the Norwegian Tourist Association
(www.dnt.no) to get advice and check for options. They have cabins spread
around in most parts of the country, many of them available for hikers, self-
serviced but free of charge. They can also provide valuable information regarding
safety issues.

Remember too bring alot of money. Norway is really expencive!

You should have a look at this site, tells you everything you need to know.

Visit norway

In my opinion as mentioned earlier, Oslo is not where you should start from.

If you would like to see fjords, I would recommend Geirangerfjorden or Hardangerfjorden, but that's like 7-8 Hours with car from Oslo.
All the fjords are beautiful and unique in their own way.

There is a lot to see of Norway's nature, some of my personal favorites are the western fjords, the archipelago of Lofoten and wast wilderness of the mountains.

I think maby Jotunheimen could be something for you, check it out. that's like 5-6 Hours drive by car.

Enjoy!

Check out this link in english
http://www.visitnorway.com/places-to...-park/?lang=uk
Rondane national park has very nice scenery