Oslo has a very easy-to-learn public transportation system and the metro system has a total of six lines. They all pass my Majorstuen, Nationaltheateret, Stortinget, Jernbanetorget (Oslo S), Grønland and Tøyen and then they travel on in all kinds of directions after that.
There are several tram lines and bus lines in Oslo as well, and if you have need for any of these you can take a look for description of the lines at the different stations.
Tickets for the bus, tram and metro should be bought beforehand. In some cases you can buy on board, but that is more expensive. The penalties are very high if you are caught without a ticket, so you should not try!
Public transportation costs:
Single ticket: 30 NOK (1 zone, for adults)
Day card: 80 NOK (1 zone, for adults)
Week card: 220 NOK (1 zone, for adults)
You can also buy an Oslo Card which will give you public transportation pass and lots of discounts and free entrance to several museums if you would like. For more information, check out “Oslo Pass” (external page).
Oslo Metro Map: