A pass which gives you unlimited travel on the bus, tram and train; and entry to many museums for one price. Available in several durations.
www.visitoslo.com
Tourist Information Offices and some hotels.
Updates
The brilliant thing about this pass is that if you get it for 72 hours at the moment you get a 390Kr cruise for free which goes round the islands (2hrs) and get a HUGE and delicious Prawn Lunch Buffet (fresh, warm bread), butter, chilli sauce and the most delicious unlimited peel your own prawn feats the eye could see!
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