seasick,
I take it that you will be working at OSL, in one way or another, and thus you will have your very own ID-badge. Take that ID-badge and go to either of the two following places;
1: SAS Flybussen --> stops outside the terminal, at ground level. If you show your ID to the driver, you will get 50% discount on single trips, or, buy a card with 30-ish trips giving you a 75-80% discount. Dirt cheap, but remember that the buses can get caught up in traffic!! ...and Oslo traffic at rush-hour is no pic-nic.
...or...
2: Flytoget (airport train) --> Go to Flytoget's desk at groundlevel, and ask them about their rebate-card for people working at OSL and show them your ID. You will then get a form to fill out, and you also have to provide a passport photo of yourself. I don't recall neither the cost of the card, or the amount of trips on it, but it too was dirt cheap.
Of the two, I would definately go for the train. Actually cheaper than the bus, and a lot quicker. Not to mention you don't get caught up in rush-hour traffic (why is it called rush-hour, when everything pretty much stands still?!
)