There are two options for travelling between Oslo and the Airport by train;
(1) Flytoget Airport Express trains
(2) regular trains operated by Vy - the national rail operator.
You also don’t have to use Oslo S station, both the Flytoget and Vy trains also operate between the airport and Nationaltheatret Station, which is located at the opposite (western) edge of the city centre to Oslo S.
When heading to the airport from central Oslo look for trains that are heading to ‘Oslo Lufthavn’.
On some timetables the airport station is ‘Oslo Lufthavn Gardermoen’.
Taking the Flytoget Trains:
The key features of using this dedicated airport train service are:
(i) The journey time between Oslo S and Oslo Lufthavn is around 19 minutes, so they're faster than the regular trains.
(ii) it's generally a very reliable service – Flytoget trains are given priority over regular Vy trains
(iii) However, tickets are more expensive than the regular trains.
(iv) They usually* depart every 10 mins in both directions between Oslo S station** and Oslo Lufthavn station.
*Every 20 mins on Saturday and Sunday mornings and during most of July.
**4 x trains per hour operate between Nationaltheatret station and Oslo Lufthavn station
When the 6 x trains per hour service is operating. two of the trains commence the journey at Oslo S station.
So if you’re heading off from Oslo S these trains, which commence their journeys there, have easier boarding.
Taking the regular Vy Trains:
In contrast to taking the Flytoget trains, the regular trains are:
(i) Slightly less regular = they depart no more than 3 x times per hour
(ii) All trains call at Olso S station and Nationaltheatret station
(iii) They're only 5 mins slower, the journey time between Oslo S and Oslo Lufthavn by these trains = 24 minutes
When travelling from Oslo look for trains that are calling at Oslo Lufthavn, but are terminating (usually) at Eidsvoll or Lillehammer.
(iv) The Vy trains are generally busier than the Fltytoget trains, particularly the trains heading to/from Lillehammer.
So seats aren't guaranteed, particularly when heading to the airport on Monday - Saturday between 16:00 and 18:00
(v) Tickets are generally (much) cheaper.
(vi) Rail passes are not valid on the Flytoget trains, but they are valid on these alternative trains.
There are also direct Vy trains between Oslo Lufthavn and Trondheim.