Flight
Oslo Airport Gardermoen (OSL) is by far the best option to fly into. From there, you can get on the express train ("Flytoget": http://flytoget.no/flytoget_eng/) which will take you to the city center in about 20 minutes.
Train
If you're flying into a different city than Oslo and want to take a train from there, this is the train stop you want to get to.
Travel Note
The ceremony itself will be held at Bekkelaget kirke, address Sandstuveien 15. Using public transport, the easiest ways to get there are a) taking bus 74 direction Mortensrud T to "Bekkelagshjemmet" (+ walk about 4 minutes) b) tram 18 or 19 direction Ljabru to Holtet (+ walk about 10 minutes)