The metro, in Norwegian is the Tunnelbane or just T-bane. Oslo has 6 main lines, one of which (Ringen) is the ring or loop line, which circles from northern Oslo to the centre and back.
All stations except three on Ringen (Sinsen, Storo and Nydalen) have separate entrances and platforms for trains going east or west.  Be sure to check the platform number to avoid unnecessary delays.