To really get around in Oslo in a practical way I propose to make an underground circle-line. Important new features are Bjørvika, Hovedøya and Phillipstad, which today has no real good connection with the rest of town.
Since I can not spend all my time on city planning, the stops suggested are of subjective choice; Departure in both directions every 5 minutes, 18 minutes one round, 8 trains in constant move
Stations from left to right:
Bislet - Homansbyen - Solli plass - Aker Brygge (Phillipstad) - Hovedøya - Bjørvika - Loenga - Grønlandshagen - Platousgate - Botanisk Hage - Birkelunden - Ila.