Visited Oslo a couple of years ago on a trip to Europe. Everything is closed Sunday and Monday, and when stuff is actually open, it closes early anyways. Everything is crazy expensive. I still remember having to pay 17$ for a beer at a bar, and 30$ for a thin tasteless bowl of soup and crackers at a cafe. Your canned soup is 3 steps above this.
One thing that was actually on the cheaper side is the loads of gravlox you can buy. If you are cost conscious (and you will be once you see your first Norwegian bill) you can buy bread and gravlox at a grocery store and make a great sandwich.
Scandinavia is notorious for bland boring food, and that's coming as a Canadian standard. You've been warned.
Very beautiful though.