The cheapest way is to use your regular ATM/debit card in a no-fee Norwegian ATM and decline dynamic currency conversion (DCC)--but only if the foreign ATM fee charged by your bank is lower than the foreign-transaction fee charged by your credit card.
DCC is always to be avoided; it crops up often on credit-card purchases, too. If you accept the offer of DCC, the withdrawal/purchase amount will immediately be converted to dollars (or your own currency) at a rate that is very disadvantageous to you. You'll end up paying an extra 3% to 7% (or could be higher). And having the transaction converted to dollars does not mean your credit-card company won't charge a foreign-transaction fee; it will still do so, because you made the transaction in a foreign country.
When is your trip? Do you have time to get new cards? CapitalOne 360 accounts have no-fee ATM and credit cards. Charles Schwab has no-fee ATM cards. So do many credit unions, and I know there is at least one other option. Some of those say they will refund to you any usage fees assessed by the ATMs you use. As a courtesy to those companies, I make a very, very strong effort not to use fee-charging ATMs. No one is going to protect you from DCC; it's on you to read screens very carefully and reject anything that offers to record or charge a transaction in a currency other than the local one.
All that said, Norway is very, very cash averse these days. I spent close to 3 weeks there last year, and I encountered only two situations where I absolutely needed cash (coins specifically): a luggage locker at the Dombas train station and a public toilet in a park in Oslo. There were far more places that were card-only, so you are quite likely to be forced to use a credit card or debit card from time to time.
Another issue specific to Norway: Since so few enterprises want cash, there's not much need for ATMs, so nearly all the bank ATMs have been removed from service. I spent a lot of time looking in Oslo. When I eventually found one, there were about a dozen people lined up to use it. I asked where I could find an ATM while transacting other business at the tourist office outside the main train station. The staffer pointed at a building straight across the square. I was appalled when I got close enough to see that there was a currency-exchange office in front of me (almost certainly the very most expensive option). For some reason I walked around the corner of that building to the right, and there was an ATM mounted in the exterior wall. I don't remember whose ATM it was; I don't think it was well-marked. But I decided I was on a fruitless mission to find a way to make a quick, free withdrawal, so I'd just get a small amount of money and accept the fee. To my amazement, that Brand X ATM didn't charge me a fee. It did offer DCC, which I duly declined, so I got some Norwegian currency with no extra charge. Which I had considerable difficulty actually spending, but I just wasn't comfortable having no local money at all.
So my advice for Norway now is that (after all least trying to get an ATM card that will not itself assess a fee) you try out any ATM you encounter, reading the screen carefully and declining DCC. You may be as lucky as I was. If you aren't lucky the first time, keep trying ATMs. But don't forget--your bank at home may charge you a fee, too, and you won't know it until you get you next ba k statement.