Oslo's international airport is a whopping 50km (30mi) north of the town centre in Gardermoen. To get there from Oslo (or vice versa) take the Flybussen shuttle bus, which departs every 20 minutes (40min). The airport express train departs Central Station every 10 minutes and the trip takes about 25 minutes. All regular intercity trains arriving from northern regions will stop at Gardermoen, as does the train from Skien.
Oslo is well served by air from most of Europe and North America. Travellers from elsewhere will need to catch a connecting flight.
Buses regularly do the trip from neighbouring countries Sweden, Denmark, Finland, Germany, Russia and, if you're a masochist, the UK. The journey from London to Oslo takes about 38 hours and flying can sometimes be cheaper - not to mention much faster.
There's an efficient ferry service between Oslo and other Scandinavian countries as well as the UK and Germany, popular for people with their own vehicles.
Trains operate between Oslo and several European cities including Stockholm, Copenhagen, Hamburg and London.
Oslo's not short on transport options. They include buses, streetcars, a subway, taxis, and ferries on the fjord. The outlying areas of the city are all easily reached from downtown. Ferry services are available from the downtown piers to other areas of the city. Longer rides south on the Oslo Fjord are also possible from the downtown piers. Perhaps the best way to navigate the city centre, and most of the sights, is by good old-fashioned foot power. Compact little Oslo lends itself remarkably well to walking.