If you are planning on doing some sightseeing in Oslo, I recommend you buy the Oslo Card. The price is not bad at all (surprisingly), and public transport (ferries, buses, trains, trams; that alone costs over 60 NOK - $10 - for 24 hours) is included, as well as the entrance fees to all the major sights and museums. If you're only going to 2-3 museums and will be using public transport for more than 3 trips, you get good value for money. See VisitOslo.com  for more info on all sights and activities in Oslo and how to get there. Also check out the travel planner in Oslo, trafikanten.no (click on the British flag on the bottom left side of the page for information in English), which helps you find the right bus/tram/ferry to where you want to go.

Viking ship museum E-mail

The beautiful ships in the  Viking Ship Museum are filled with tools, jewellery, sledges and more were excavated over a hundred years ago and are the best preserved viking ships in the world. A definite must when coming to Oslo! Be sure to also get the ferry ride from the harbour near city hall, as well as the other museums on the quiet Bygdøy peninsula.

 
Are you spending summer in Oslo?
 
Have you tried the Oslo Pass yet?