The region is delimited by mountains in the north and west, by the Swedish border to the east and by Viken and Skagerrak to the south. The border towards Sorlandet is less pronounced.

The mountains reach a height of 2469 metres in the Jotunheimen mountain range, the highest point in the Nordic countries (excluding Greenland). Other prominent mountain ranges include part of the Dovrefjell in the far north of the region, the Rondane north east of Lillehammer and others. The high plateau of Hardangervidda extends into Western Norway.

Valleys cut deep into the mountains, from east to west the main valleys are Osterdal, Gudbrandsdal, Valdres, Hallingdal, Numedal and the valleys of Telemark. Osterdalen is surrounded by mostly flat areas of conifer forests, but the others are all deeply cut into the mountains.

The area around the Oslo fjord and towards the north east are comparatively flat, and there are patches of intensely cultivated lands, notably Hedmark, Toten, Hadeland, Ringerike and others. The population density in the flatlands is the highest in the nation, and some 40% of the nation's population lives within 200 km of Oslo. Numerous islands shelter the coasts, creating a paradise for swimmers and boaters in the summer.