After a little search to find the tourist info (just around the corner from City Hall with the bus pickup directly across from City Hall) I purchased the All Inclusive Tour for 380NOKs. Since it is January the bus had fewer than a dozen tourists. The bus was clean and comfortable with clean windows that were not scratched (important detail). Our tour guide was Sylvia, and she was very personable and informative. The tour includes most major locations, with stops at Vigeland Sculpture Park, Holmenkollen Ski Jump, the Viking and Fram Museums and the Opera House. Note the bus does return to the original pickup mid-tour for those who bought a lesser trip which does cause a bit of backtracking. Museum entrance was included in the price too. I had only two days in Oslo, and I am so glad I chose to spend most of one with this tour. By the way, do not miss the City Hall interior (free) where the Nobel Peace Prize ceremony is held. The Peace Museum is also right across the street near the harbor.
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