Are you doing a trip into a fjord while you're in Bergen? Especially if not, you could spend a night or two in fjord country on the way back from Bergen. That seems like a good use of one or two nights since you have extras. I think I could amuse myself for six nights in Oslo, but I like to spend a lot of time walking through different neighborhoods, and I enjoy museums, and few others have suggested spending that much time there.
I have notes put together for a trip to Scandinavia that didn't happen in 2020. They say the town of Frederikstad is pretty. It has a nice waterfront and an historic fort. It's a popular weekend getaway for folks living in Oslo. The train takes about an hour.
Another possibility would be some sort of boat trip on the Oslo Fjord, though it isn't as dramatic as the ones farther west. The fjord town of Drobak is said to be nice. Boats go there, but sporadically, so bus would probably be better. The information I have is that Bus 541 or 542 would get you there in about an hour; and there are two buses per hour, departing from behind Sentralstasjon. Tickets are cheaper if you don't buy them from the driver, but my notes are silent on precisely where one should buy them.
Finally, this is from my notes on Lillehammer. I haven't been there, so this could be very wrong:
Worthwhile only for excellent Maihaugen Open-Air Folk Museum. Norwegian Olympics Museum has sparse English and is just a back-up rainy-day activity. City does have “two happy, old, woody pedestrian zones (Gagata and Storgata)”. Gudbrandsdal Valley not as scenic as fjord area to west. Fun eateries in town center (try Elvegata on both sides of Gagata) are better food sources than at Maihaugen.
Good luck with your trip. Norway's so expensive that it's smart to plan ahead so you can use your time efficiently. I'm usually a wing-it traveler, but I won't be doing much of that in Norway.