Kolsås is a wooded mountain ridge in the municipality of Bærum, Norway. Geologically, Kolsås belongs to the Oslo Graben area. Its two peaks consist of hard rhomb porphyric lava covering softer rocks, forming steep cliffs to the east, south and west. An old farm beneath the mountain has the name Kolsberg.
The first element in this name is the genitive case of the old male name Kolr, and the last element is berg n 'mountain'. The parish and municipality of Bærum (Old Norse Bergheimr) is probably named after this prominent mountain. The last element in the name of the mountain was later changed to ås m 'mountain ridge' to distinguish it from the name of the farm.