My wife and I spent a couple of days there last summer. They were the last few days of a three-week long trip and, unfortunately, I came down with a cold.
Must-dos on my list would be to visit Vigeland Park and the Nobel Peace Prize Museum.
I have photos on my home page gallery. Oslo starts here:
http://www.spotstop.net/gallery/Norway?page=5
(We enjoyed Bergen more, too.
http://www.spotstop.net/gallery/Norway?page=1)