The best way to travel from Oslo airport (OSL) to NIH would be to take the Airport Express Train to downtown Oslo (Oslo Sentralbanestasjon) and then take a taxi (address: Sognsveien 220).
You may also take metro no 5 (direction Sognsvann). The metro leaves every 15 minutes and the trip takes approx 15 min from Oslo S/Jernbanetorget. Get off at the last stop, the school is very close by.