Here is a Google Earth view of the old school on Hoffsveien: OAS site until 1960

The small building in the top center of the photo is today the Linden Rudolf Steiner Barnehage at Hoffsveien 41. OAS was downhill from that & on the other side of the creek.

In this 1960 photo from Anita Messier, there is an apartment building in the background that is still standing today. You can walk downhill from the barnehage and view the apartment building from the same perspective, at which point you are also overlooking where OAS once stood. The style of construction of the barnehage resembles that of the old school visible in the background of this picture and may once have been part of the same complex (reportedly a former German prison barracks).

There was an even older incarnation of OAS on Oscarsgate in central Oslo. I'll have to look up the address later.