Yup. Our summer holiday was in Norway; it was pretty much a tour but we started and ended in Oslo – we were there for a few days at the end.
First thing is prepare yourself for the expense. And, as above, the weather this time of year.
The opera house is spectacular and well worth a visit. Plenty of museums to suit all tastes, including several on islands you can get to – but then I’m not sure if they run all year round. Shopping, even just browsing, should be awesome this time of year. Tourist information should be able to arm you with all the brochures you’ll need.