There's really no point in camping outside the field. If you want to get up front and kiss Mick, you should be there in line around noon. There is a big field outside the stadium where you can relax with a few beers (just bring them yourself).
Last time I saw them there, we went in just before the support act started, and we got a nice place to see with no hassling or pushing. Stones fans are really nice.
To get there take the subway number 3 (eastbound) from Stortinget, and get of at Helsfyr station. It's a 10 min. ride. I guess there will be a few others going, so just follow the crowd.
Hope this'll help you. Have a great time, and we'll see you there.