(check here for information about public transportation in Oslo)
Transport from/to Oslo Airport Gardermoen 
There are two Airports for Oslo when you order fligts via web. One is Oslo Airport Gardermoen (OSL), and the other is Sandefjord Airport Torp (TRF). TRF is far from Oslo center (110 km south-west of Oslo). If you don’t have plans to travel around before or after the conference, don’t use this airport. There are cheaper charter flight tickets from other European cities to Torp, but please note Torp (TRF) is very remote from Oslo.
Main Airport for Oslo is Gardermoen (OSL).
Information about transport to/from Oslo Airport Gardemoen is here:
http://www.visitoslo.com/en/transport/transport-airport/oslo-gardermoen/
Don’t use a taxi from the airport. It’s really expensive and it takes time. If you have a lot of luggage, take the train to Oslo S first and then take taxi from there to your hotel.
Trains:
Airport express (Flytoget) has several departures and is convenient. It takes around 20-25 min. to City center.
Price between Oslo Lufthavn (Gardermoen) and Oslo Center (Oslo S or Nationaltheatret) 180 nok. If you are student with the student ID or Senior over 67 years old, the price is 90 nok. And your partner (wife, husband) of the senior person also can use 90 nok ticket. Remenber to bring ID for student or senior.
http://www.flytoget.no/flytoget_eng
NSB (Norwegian State Railways) has local trains to Oslo Center (Oslo S or Nationaltheatret) . Departure around twice a hour, depend on the time of the day. Price is 93 nok. Senior 47 nok. It takes around 20-25 min. to City center.
https://www.nsb.no/en/frontpage
Busses take longer time, but sometimes drive to your hotel directly. So if you stay one of them, they can be convenient.
See «Bus to/from Oslo Airport» on this site: http://www.visitoslo.com/en/transport/transport-airport/oslo-gardermoen/