Main sights of Oslo:
Most of the main sights are within walking distance of the city centre, like the National gallery, the Opera house, the Royal palace, the City Hall & the Akershus fortress. We recommend a scheduled city sightseeing to see the sights outside the city centre, like the Holmenkollen Ski Jump, the Fram & Viking ship museums and the open air Folk Museum at Bygdoy penninsula.
You can easily take a tram to the Frogner park / Vigeland sculpture park, but this park is also included in some of the city sightseeing trips. The Munch Museum at Toyen is easily reached by the underground.
Read more about Oslo here