I think the answer is probably it depends. I went through OSL twice in the last week arriving at the same time on the same flight both days; one day I was first off the plane and there was literally not a soul there other than the staff when I got to customs and immigration, the other day there was a queue of well over a hundred people (it snaked so far around the corner and down the ramp that I couldn't actually see how long it was). I think it's very dependent on if you arrive after a longhaul flight or not. There is a connections desk that goes through to a walkway but it doesn't always seem to be staffed so that you have to ask someone to open it for you. Personally, I wouldn't risk it!