Avoid using the Airport Express Train (flytoget) from the airport to the city. Instead use the local train which is half the price and takes only an extra six minutes.
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Far the best are the Flybuss (express buses), cheaper too. They leave from outside the airport main exits. But here is typical Norwegian confusion. Both buses have number B15, enter and leave Oslo at different places, and leave at the same time, every 15 and 45 minutes over the hour.
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