An excellent city for a day trip.

Take a ferry or bus over to Bygdøy and visit
both the Kon-Tiki and Fram museum which
detail the extraordinary lives of 2 of Norway's
great sons, Thor Heyerdahl and Roald Amundsen.

Equally as fascinating is the Norwegian Resistance Museum
within Akershus castle's grounds which chronicles the brave
exploits of many a brave Norwegian as well as the incredible
treachery of Vidkun Quisling whose name is now a synonym
for perfidy.

I wouldnt waste too much time over lunch. Oslo is horrendously overpriced.