Here's a JFK/OSL roundtip for $397: https://www.google.com/flights/#search;f=JFK;t=OSL;d=2014-11... (If the link doesn't work, it's an outbound Friday November 28, return Tuesday December 2, on Norwegian 7002 and Norwegian 7001.) This may be a Thanksgiving-related dip that makes it extra-low, but it's not much higher on some other dates nearby.
When you go to buy the ticket, the price may not match, so be aware of that. I think it's usually right, though. I bought a similar flight at a similar price, so I can sort of vouch that it's not some garbage quote. Google Flights is pretty good for this, at least if you're willing to tinker with the dates and cities until you find the magic spot. They have a little bar chart of the prices, so you can look at surrounding dates and find better deals. I don't know if I'm just doing it wrong, but the other flight search sites I've tried sometimes can't find the deals on Google, even when I search specifically.
Also, consider booking on www.norwegian.no for probably ~$30 savings, but it's only in Norwegian, so if you can't read it confidently, maybe that's a little too crazy.
When you go to buy the ticket, the price may not match, so be aware of that. I think it's usually right, though. I bought a similar flight at a similar price, so I can sort of vouch that it's not some garbage quote. Google Flights is pretty good for this, at least if you're willing to tinker with the dates and cities until you find the magic spot. They have a little bar chart of the prices, so you can look at surrounding dates and find better deals. I don't know if I'm just doing it wrong, but the other flight search sites I've tried sometimes can't find the deals on Google, even when I search specifically.
Also, consider booking on www.norwegian.no for probably ~$30 savings, but it's only in Norwegian, so if you can't read it confidently, maybe that's a little too crazy.