This is actually a tricky question. According to the regulations (law) found here
http://www.lovdata.no/for/sf/sd/td-20040119-0298-008.html
there are certain conditions that must be fulfilled in order for foreign driver licenses to be recognized by Norwegian authorities. The main rule seems to be that the issuing country, if it is not Scandinavian or a none EU/ECC member, must have ratified either or both of two UN conventions on road traffic. And the national driver license must be designed according to those conventions. If the design does not fulfill the requirements described in the conventions, a license from a ratifying country will still be recognized if it is accompanied by an international drivers permit, such as it is described in those conventions.

How any of this affects US issued licenses I do not know. Since the US Department of State has authorized two American car organizations to issue IDPs, the US obviously is a ratifying country. So a US drivers license might very well be valid in its own right in Norway for short time use. I admittedly was a little haste full to draw conclusions there. But either way it does not hurt to obtain an international drivers permit to be on the safe side. They do not cost very much. And is highly recommendable. Even in a case where it is not required by Norwegian authorities certain car rental companies might.