You could drive a car in Oslo, but I wouldn’t recommend it. Parking is very expensive and often limited to a just a few hours. If you’ve bought an Oslo Pass, you can park free on public lots. But even so, driving can still be challenging (and annoying), as streets are often one-way or closed for through-traffic.
Instead, public transport is a good option in Oslo; the second best to walking. A system of trains, busses, trams, metros and fjord ferries cover the entire capital area as well as towns and cities further out. With the Oslo pass, public transport is free. But otherwise, rule number one is to buy the ticket before you get on. On the metro, it’s the only option and if you’re caught without a ticket you might find yourself heavily fined. On other forms of transport, you can buy tickets on board, but it’s more expensive.
You can buy tickets and flexicards at stations, or at Narvesen or 7-Eleven kiosks. Tickets are valid on all forms of public transportation, including the scenic Oslo fjord ferries. Here’s information on fares. You can check schedules at Trafikanten (click on the British flag for info in English).
Fun tip: check out the Tunnel of Light at Nydalen metro station, incorporating architecture, light and colours into the escalators.
Another great option in the city centre is the hop-on hop-off mini cruise on an old wooden sailing boat in Oslo harbour, stopping at Oslo Opera House, Bygdøy and City Hall (next to the medieval Akershus Fortress and Castle, Aker Brygge and the Nobel Peace Centre).
If you would like to venture out of town, NSB trains will take you to all neighbouring cities and further: to the UNESCO-listed city of Bergen on one of the world’s top 25 train journeys, across gorgeous Mount Dovrefjell to the old Viking City of Trondheim or even as far as Bodø, north of the Arctic Circle.