Any vegetarian/vegan coming to Oslo should check out Blitz. This place has been around since 1982, and serves the cheapest vegan and vegetarian food in all of Oslo. Lots of punks/alternative people go here to hang out or practice with their bands. Blitz offers open faced sandwiches with a huge variety of toppings (omelette, fried tofu on hummus, potato burger, sautéed mushroom with cashews, different cheeses ++), baguettes with tofu or cheese with hummus and greens and so on, and a new main dish every day. The main dish isn't always vegan, and not always spectacular, but always delicious, and it only costs 40 kroner (5 euros)! Everything's made by volunteers, and the selection will vary according to who's working that day. Try the waffles, they're world famous (in Oslo).Pros: Excellent and super cheap food, The atmosphere and crowd, Huge portions!
Cons: Not always a lot of gluten free options.