It depends on how tight your budget is exactly. Expect 250-300 NOK for a hostel bunk. If you go to a club on the weekend, there's often an entry fee of 50-100 NOK. Pubs don't have cover charges unless there's live music.
A half liter of beer costs 55+ NOK, usually more. A bottle of wine in most pubs or restaurants is at least 300 NOK. Sightseeing can be cheap as long as you're just walking around the city.
Museums and other sights vary in cost, maybe 50-100 NOK. For sightseeing recommendations:
http://wikitravel.org/en/Oslo
Food is incredibly expensive, even in a supermarket. Stick to the cheaper shops like Kiwi, Bunnpris and Rema 1000 (less choice, but also less expensive) or immigrant stores which usually have very cheap vegetables/fruit that look a lot better than the chain stores. A loaf of bread can cost as little as 7 NOK, but usually closer to 20 NOK. I don't know what you plan on buying to eat so I can't give you exact prices. I spend at least twice on food as I did in the UK, and 3-4x what I did in the US. You need at least 100 NOK per day for grocery store food (bread, cheese and some vegetables for sandwiches) but it depends on what/how much you eat.
Currency exchange rate:
http://xe.com