Located at the edge of the Nordmarka, sprawling Bogstad Camping is an ideal base for enjoying Oslo outdoors. The facilities include showers and communal kitchen, and there is a nearby kiosk and restaurant. It's 9km north of the city centre. To get here, take bus 32 from Oslo S (about 30 minutes).
If you don't have a tent or camper, there are cabins as well as furnished glamping-style tents.