Though Oslo is considered one of the worlds most expensive cities to live in you’ll easily be able to get by if you’re on a student budget. You’ll find a lot of Asian restaurants as well as pizzerias in most parts of the city, but if you’re in the city centre you should check out the Grønland area (metro stop there as well). For more on where to eat:

http://wikitravel.org/en/Oslo#Eat