Bus or tram: From Oslo Central Station you can take tram no. 17 or 18 in the direction of «Rikshospitalet» to Adamstuen tram station, use the tram station «Jernbanetorget». Check out Ruter.no for other public transport suggestions.
Car: There are several routes to take. In terms of traffic, it is often easiest to drive E6 /Ring3 if you are driving into Oslo, rather than venturing the often heavy traffic through the city centre. Check out Google Maps for the best route for your purposes.
From Oslo Airport: From Oslo Airport (OSL) airport express trains depart to Oslo Central Station every 10. minute. For further transport from Oslo Central Station, see information on buss and tram above.