We visited this museum on our last day in Oslo. It's was a national holiday (Monday after Easter Sunday) so we weren't sure if it would be open but happily it was.
As it's free to visit and everything in Oslo is rather expensive, I wasn't expecting much. However, the museum is huge and has so much to see in it. I'd give a good couple of hours or more to properly take everything in.
It takes you right through from around the 16th century, through to modern day. It has great exhibitions on the Napoleonic era, Norway's independence, the Second World War, the Cold War and then more modern war.
There is so much to see with lots of information in both Norwegian and English. Just be warned in some areas (the Napoleonic era) you need to scan a QR code with a smart phone to get other language information as they only have Norwegian information boards.
I'd definitely recommend this even if you are not really interested in notary history.More