Delta Airlines offers great solutions for travel to California from North Europe. For example you can book a multi city flight ticket from Oslo to Los Angeles and back from San Francisco to Europe for only €362. Stopovers in Amsterdam and Paris.
Travel dates: Februar to April 2015.
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Bookable on Kayak.
Between Los Angeles and San Francisco I advise you to travel with Megabus. This low cost bus company has great fares starting at $5!
You can get to Oslo from many European cities with low cost airlines, such as Wizzair, Ryanair and Norwegian. From Budapest a direct flight costs only €38.
For your information: On the way back to Europe there is a stopover in Paris, where you can simply skip the last leg to Oslo and get off the plane if you want and take an another flight from Paris to your home. However you have to add Oslo as a final destination because this is the only way to reproduce this fare! It is not possible to skip the first leg Oslo – Amsterdam, your ticket would be cancel then.