If you want to buy tickets to fly from Copenhagen to Oslo or Bergen to Copenhagen on Norwegian Air, can you do it more cheaply once you get there, or do you have to do it before leaving the U.S.
i buy my airline tickets before i go and most of the time way before i go.
i would be interested to see what others say about doing the "wing it".
what you can do is to look at buying tickets "tomorrow" and then 3 months from now and see if there is any price difference. But that may answer your question too.
I like Ray's advice about checking prices on different dates. Some airlines provide a low-fare calendar. For most intra-European flights, you buy the tickets from the same web site whether you are here or there.
I wouldn't expect the price to go down. I bought a ticket on Norwegian for an August trip back in January. I paid about $50. I just checked the same flight and the price is now $192 (and it is still 3 months out). If you see a cheap price and know you want to travel that route, then I'd suggest buying.