The Dubliners pub on Rådhusgata is the best and probably only place in Oslo for folk music. There's session on Saturday afternoon from 4pm in the front bar followed by a pub band from about 10pm. on Tuesday nights there is a session in the back room from 8:30pm. Saturdays is more tune based and Tuesday a bit quieter and more song based but both "genres" are welcome to both.
If you want to go to a few museums then there is an Oslo pass which will give you access to all the museums and free transport on buses, Metro, trams and ferries. It's not cheap but it's worth it. I would certainly recommend the Fram and maritime museums and the museum of Norwegian life; all of which are on Bygdøy and can be reached by a short ferry hop.
It's a fun city, I hope you enjoy your visit!