Norway is expensive, but as Dzug infers not as shockingly so as it once was (as our own taxes have gone up so much in recent years).
Like anywhere though, you can do things to make it a bit cheaper. For example rather than say going to a cafe/restaurant for lunch, buy the 'components' from a deli/bakery and make your own. think creatively and you can save a lot.
In Oslo they have something called the 'Oslo Card'. This covers admissions to several attractions over a number of days. I remember it as being very good value for money (unlike the equivalent scheme in New York).
If you enjoy a tipple in the evening, take your maximum duty free allowance with you. Spirits from a shop can run to about �80/litre, you can imagine what it costs out. All wine is of course imported, and starts at around �40 a bottle for house wine (in a restaurant). Beer in an everyday bar will be �5 and up a half litre.
Go to the nearest Spar, load up on mixers. Empty the hotel room fridge, and keep your mixers in it. Enjoy a cocktail or two in your room before heading out in the evening. That alone should help axe your restaurant bills...