Oslo is ranked as one of the best coffee cities in the world and you will find cafés serving good coffee everywhere. Serious chains like Kaffebrenneriet and Stockfleths are worth a visit, but there are numerous other great places. To mention a very few:

Tim Wendelboe, Grünersgate 1 (Tram 11, 12 and 13 to Olaf Ryes plass). Tim Wendelboe is a micro roastery and espresso bar in Grünerløkka serving what's considered by many the best coffee in the world. Tim Wendelboe means business. It's a tiny establishment and seating is scarce. If you want a simple black coffee you will most probably be pointed to a small lab-like room where the coffee will be made for you. So if you like great coffee this is an obligatory stop. They also arrange barista courses.

Fuglen, Universitetsgata 2 (Tram 11, 17 and 18 to Tullinløkka). To enter Fuglen is like enter a living room from the 60s. Not only do they serve great coffee, but they also sell retro furniture and vintage design. In the summer of 2012 Fuglen also opened a branch in Shibuya, Tokyo.

Mocca Kaffebar & Brenneri, Niels Juels Gate 70 B (Tram 19 to Riddervolds plass). Good coffee in Briskeby, Frogner.

Java Espressobar & Kaffeforretning, Ullevålsveien 47 (Bus 21 and 37 to St. Hanshaugen). Great espresso bar located next to the St. Hanshaugen park.