We were in Oslo for about 7 hours on a cruise in July and thought it was a wonderful city. The Town Hall is magnificent and I would urge anyone to visit. There are guided tours in English at 10a.m, 12 noon and 2 p.m. - geared to the times for the cruise ships arrival and departures I would guess. We also managed a quick visit to the National gallery which deserved a more leisurely pace. Then we scooted by ferry over to the Viking Museum - fabulous. (If you are retired you get in half price btw.) For transport you have to be over 67 though!
We are already planning to go back on a city break, as there were places we would have loved to see but didn't have time on this taster trip.