A small tip on the transport to Oslo from the airport.
Regular oneways are 170 NOK. 50% for students up to a certain age, and for seniors from a certain age. Buy the tickets from the machine, and if you are uncertain about the age requirements go for it. Hard to blame a tourist isnt it?
Havent seen any controls during my 10 years of frequent travel on this train. Only use the ticket in the barrier at the airport only, not at other stations.
You could also do regular train on the next platform, bit slower and more infrequent though. But roughly down to the price of discounted express train tickets. Same with buses, which might take you nearer to your hotel or destination.
BTW- remember there are 3 Oslo airports.
1. OSL Oslo Gardermoen which is the main airport about 45 km north of Oslo.
2. TRF Oslo Torp (located by the small city of Sandefjord some 110 km southwest of Oslo)
3. RYG Oslo Rygge (located some 80 km south/southeast of Oslo
2 and 3 are mainly for low cost carriers (and charters) such as Ryanair and Wizzair + also Norwegian has lots of flights from RYG, now also starting from TRF in addition to their network from OSL.
However be aware that there are some flights like KLM from AMS flying also to TRF. Last time I took that evening flight there was no more public transport to Oslo that evening. Might have changed now, but do check in advance.
From 2 and 3 you can take express buses or regular trains via a short shuttle.