You can expect signage and staff advising where to go at Oslo S for the replacement buses. They had the same set-up last summer, as the station (and the tunnel under Oslo) was closed for rebuilding then.

The tram stop corresponding to Prof. Aschehoug plass is called Tinghuset. The stop for Oslo S is called Jernbanetorget (literally Railroad Square/ Railroad Market Place). Note that there are two separate tram stops at different streets at Jernbanetorget. You'll want the stop in front of Clarion Hotel Royal Christiania, not the one in front of the station if taking the tram from the station. From the station to Tinghuset, you can take trams no. 17 and 18 marked Rikshospitalet, or 11 marked Prof. Welhavens gate (usually Majorstuen). In the other direction it is Kjelsås, Grefsen stasjon or Ljabru. With no luggage I would walk from Bondeheimen to Oslo S, as it is just as quick. Stops are announced on displays onboard the trams