Ryanair plans to starts a new daughter airline that will fly to 10-12 destinations in the United States within the next 3-4 years, as soon as there are available cheap airplanes to buy.
The tickets will cost only 1/10 of the prices Scandinavian Airlines take. Source: The swedish newspaper Aftonbladet
UPDATE: The route was not successful, and Ryanair has stopped this route. The cheapest routes between Oslo and Stockholm are with Norwegian Airlines now. It is also possible to take SJ trains.