Oslo is not for those on a tight budget. Even food in supermarkets is expensive, so if self-catering make sure you have some of your own supplies from home - the money saved is worth it!
Updates
The comment of an American I hostelled with was it is like shopping at the Airport Duty Free for your groceries. Not far from the truth. Coop (local supermarket chain) Xtra brand products were cheap if you can hunt them out. REMA 1000 is the cheapest supermarket. I would avoid ICA.
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