Akershus is a small county, which surrounds Oslo. The largest towns in the county are Lillestrom, Jessheim, Sandvika, Asker and Ski.
In Akershus and Oslo, the public transport is managed by Ruter.
It is cheaper to buy your ticket before you board. Tickets can be purchased at ticket machines, ticket outlets and at Narvesen, Deli de Luca, 7-Eleven and MIX kiosks.
All trains (not Flytoget / Airport Express Train) run by NSB. Buy your ticket before boarding the train to avoid extra fees. Remember that single tickets must be stamped by a conductor before it is valid.
Tickets from Router can be used by NSB and vice versa if both departure and destination are in Oslo or Akershus.
Read more about tickets and prices here.