Oslo can be done in two days, but you will be busy. One full day could be spent on the museum island of Bygdoy. Another split between strolling the center of the city and going to Frogner Park.
Bergen has a cool, nautical vibe, but it's smaller than you think. That photo you always see of the old wooden buildings on the warf is pretty much all there is of the old historical stuff. A walking tour of that, plus the Henescetic (sp?) tour a few doors down, and the fish market is one easy day. Beyond that, the tourist sites fade fast.
So if you can devote two FULL days to Oslo and one FULL day to Bergen, then my first choice for what to do with that extra day would be to spend the extra day mid-Nutshell. Read up on that chapter in the Rick Steves book and also Lonely Planet, and then pick a spot & do some hiking.
My second choice would be an extra day in Bergen, seeing some lesser sights and doing some hiking.
By the way, The Rick Steves Oslo chapter only gives luke-warm reviews of the Norweigian Folk Museum on the island of Bygdoy, but I want to tell you I thought it was wonderful - better than the similar one in Stockholm. Oslo's seemed more authentic to me. It has a nice stave church included with the rest of the buildings. The nearby Viking ship museum was great, too - very simple and elegantly done. While in Bergen, we also took a bus a couple miles out of town to a stave church in the woods. It was a reconstruction, but interesting.