From Sørkedalen school:
This is the shortest route. Easily accessible with public transport. Take metro to Røa and change for bus #41 to Sørkedalen skole. Check schedules and routes at Ruter.no. If you are driving, there is a big parking lot at Elveli. From the bus stop, follow the marked trails by blue signs. You can either follow the road, or the path in the forest. If you want to go by bicycle, it is easiest to follow the road. About 5 km in total and 250 meters in elevation. Expect around 1,5 hours up, 1 hour down.
Take the metro to Frognerseteren. From here, follow the blue signs to Kobberhaughytta. About 8 km to walk, expect to use about 2 hours each way.
Take the metro to Sognsvann. Follow the blue signs towards Kobberhaughytta. You have several options on the way, depending how far you want to walk, and if you want to follow trails or road. Expect about 10 km and 2,5 hours each way.