Blitz is unique, not just in Oslo but I'm pretty sure in the world. If you go in there expecting a 'professional' or 'high class' experience, you'll be disappointed but if you read anything about Blitz, you won't. Blitz is a non profit cafe run by volunteers and punx with mismatched crockery, tea for 5kr-, great company, graffiti everywhere and the ability to smoke indoors. The food is simple and cheap with at least one vegan option. If you think you'll turn your nose up at somewhere like this, it's not for you. Punk lefties will love it.
Cons: Just wish it were open on Saturdays