This is an official free beach by a small forest lake with bog water east of the somewhat larger lake Sognsvann in the northern outskirts of Oslo.
Take subway number 5 to the end station Sognsvann, or take the car and park in the big parking lot. Walk along the gravel road in direction "Ullevålseter" for about 15 minutes. When you see a stone dam on your right hand side, you are there!
Some sloping rocks, nice evening sun. The beach is not well suited for small children. There is a toilet in the vincinity, close to the lake Sognsvann.