Oslo is such a great city to visit!! Norway was not really on my list, but we were able to get VERY cheap flights. Everyone was super friendly. We went at the beginning of March, so it was still pretty cold. It was a great bonus to the atmosphere. Not much is open in the morning, so maybe do some research before you go.
Insider Tip:We went sledding and it was so much fun! You can rent pants and a jacket with your sled if you're not prepared and dressed. It was so much fun and would be a great thing to do in Oslo in the winter/spring. The metro is really great. Easy to use, cheap, and if you take it a bit out of the city, you can take it up a mountain! It is a pricey city, so just be mindful.