After some hours searching for plane tickets at Finn, Kelkoo and Skyscanner I have found out how to find the cheapest tickets from Oslo to Milwaukee. I have tried these tips on other flights as well (such as NYC, Amsterdam and Bankok) and they really worked for me:
1. Choose the right day to fly: Generally, the cheapest day of the week is Wednesday. The second best days to fly on are Tuesdays and Saturdays. This is probably becuase people travel less on these days.
2. The first flights in the morning are often the cheapest ones: This will make you arrive in Milwaukee earlier, but it might be pretty uncomfortable if the departure is at 4.00 or 5.00 in the morning.
3. Dont buy the tickets from a Norwegian page: I saved more than a thousand NOK by buying my ticket from US Airways directly, paying in US Dollars (5,60NOK)
4. Roundtrips are often more cheap than a one way ticket: a single ticket bought now would cost more than ten thousand NOK, but a roundtrip costs about 6 000NOK. I do not know how the flight companies will earn on this, but it is in my favour so I will not complain.
5. Book in time: Some might say that if you just wait a littlebit, the flights will be cheaper. I have done a little fare-compare and found out that this is not the truth. Try booking as early as possible, maybe even the day that you get the International Travel Guide from Cardinal Stritch.