Bus 24, 28, 31, 31E or 36E towards Snarøya, alight at Telenor Fornebu (Fornebuparken).
From Oslo Airport Gardermoen
Take the airport train to Lysaker, then change to bus 24, 28, 31, 31E or 36E to Telenor Fornebu (Fornebuparken).
From Oslo S (Central Station)
Bus 31 or 31E to Snarøya from Jernbanetorget.
Tickets for public transportation is best bought before you enter the bus/train. Fornebu and the central station is in the same zone (1) so a single ticket should cost 30 kroner is pre-bought, 50 kroner if bought on the bus.
More details on Telenor Expo website