Oslo has a fantastic public transportation network, with local trains, metro, trams, buses, and ferries. There are even public bicycles! It might take a bit of studying to get familiar with the system.

First and foremost, use Ruter to plan even your shortest Oslo city travels in advance. This website has an English translation available if you do not speak Norwegian. All you need to do is plug in your start and destination, then the website will produce multiple routes for you. You can specify your departure time or desired arrival time. Your locations can be addresses, transportation stops, or areas of town. You can fine tune your search so that it excludes or includes specific modes of transportation. It's amazing! This website includes all modes of transportation, including the ferries to the islands in the fjord.

www.ruter.no/en

If you did not manage to plan your route in advance or simply need to change things on the fly, Ruter also has an extremely useful, recently overhauled iPhone/Android app which can be downloaded from their website. You can also download an app that allows you to puchase tickets on the fly, called Ruter Billet.

 The train system also has a fantastic website. You can plan your travel and/or purchase tickets from the NSB website: http://www.nsb.no/frontpage/.  NSB also offers apps for iPhone and Android.

 The Flytoget airport express train has its own website for travel planning and ticket purchases: http://www.flytoget.no/eng/.

The Oslo city bike system is extremely popular for those living and working in the city. There are over 200 stations where you can pick up and drop off the bikes. Bikes are available for periods of 45min, if you want to use them longer just return your bike and pick up a new one. Their website also allows you to check where bikes are available, as does an app that updates live-time. The full season pass (April-November) costs 399kr or you can buy a day pass for 49kr. OsloBysykkel.no