From/to Oslo Airport Gardermoen 

The airport is serviced by the Airport Express Train, airport bus and taxis

Airport Express Train
The first departure for the Airport Express train from the airport is 5.30 a.m. and the last is at 00.50 a.m. Departures every 10 minutes.
Price: NOK 170,-
Travel time: 19 minutes to Oslo S

For more information: Airport Express Train

Airport bus
The first departure of the airport bus from the airport is 5.30 a.m. and the last is at 01.00 a.m. Departures every 20 minutes. 
Price: NOK 160,-
Travel time: 40 minutes to Oslo Bussterminal

For more information: Airport bus 

Taxis
Taxis are available during airport opening hours, expect a short wait during certain periods.
Price estimated: NOK 850,-
Travel time: 32 minutes to Hotel Royal Christiania