Hi. I am Norwegian so should be able to give you a good answer
The answer depends of course on which part of the country you are visting. My reply is gonna assume that you will be on the west coast or around Oslo (south of Trondheim)
You can expect temperaturs between 0 and 10 degrees celcius, colder during the nights/evening. October is a month of frequent rain and winy conditions, but will also give you some awesome clear autumn days. Only on rare occasions do we get snow or temperatures below 0 degrees in october. If you have planned a lot of outdoors sightseeing on the west coast I would also bring a full set of light rainclothes (wind and rain = umbrella no good at all)
Recommended clothing for sightseeing / city walking: Jeans, woolen/fleece sweater, shell jacet (wind and waterproof goretex jacket) - bring gloves and a good outdoors headwear (can easily be bought in Norway upon arrival)
have a nice stay!
[ Edit: Edited on 03-Aug-2009, at 02:33 by schulstok ]