Norwegians are notoriously informal, particularly in clothing. Norwegians don't usually dress smart for work and rather casually most of the time. Although some may dress up for a restaurant visit, casual dress is fully acceptable virtually everywhere.
Norwegians find it perfectly natural to wear sport clothes and rucksack anywhere.
Norwegians generally don't dress very formal. Blue denims are used everywhere. if you somebody with a fashionable suit and tie (in the midle of the day), it is probably a real estate or stock broker.
Norwegians don't hesitate to strip down to bikini or short pants in warm, sunny weather. Don't be surprised to see shoppers in bikinis and short pants only.