The NSB train from the Oslo Airport Gardermoen (Oslo Lufthavn) to Lysaker. There are two trains each hour and it takes about 23 minutes. The price for one way ticket is 90 NOK (16 USD; 12 EUR). The tickets can be bought in the red machines that stand before the stairs to go down to the trains.
Airport Express Train
The Airport Express Train (Flytoget) from Oslo Airport Gardermoen to Lysaker (trains heading to Drammen stop at Lysaker, trains leave every 20 min).
From Lysaker you take bus nr. 250 (green bus) to Grønlia. When you arrive at Grønlia, cross the road and walk towards the sea and Holmen Fjordhotel.
Check the bus schedule here: https://ruter.no/