where to get an eye exam at retail shops (not a doctor) selling eyeglasses

Kai,, watching HOURS of video on your laptop, or even on tv. is not good for your eyes. It makes them tired. Don't assume you need glasses just because of eye fatique,, getting glasses you don't need can hurt your eyesight MORE ,, so yes, get them checked, but I think you should pay and have them seen by a doctor, not an optometrist, one has more of an interest in "selling" you glasses ,, that you may not need.

There is an old saying " Doctor , my arm hurts if I do this" doctor replies - " then don't do that"

Good luck.

I can't help you with locations. But I would just ask a local to recommend an Optometrist who speaks English or the US consulate may have a list. Even if your not on the national health plan, any doctor will happily check your eyes, and at a fair price. I doubt there will be much of a price difference between a Lens Crafter type shop or a regular doctors office. Both do exams and sell glasses.

What happens if you spend hours reading a book?

Kai, this may seem obvious, but just ask someone who lives in Oslo (e.g. at your hotel, in a shop, etc.). It is highly likely that any optometrist in Norway will speak English well. Otherwise look for an "Optikk".