We plan to rent a Hertz car from a self service kiosk at Oslo Central station and return it to Bergen (BGO) airport at 8:00 am on Saturday before our 10:00 am flight. Hertz does not open until 9:00 am. Is it possible to return a car at Hertz BGO before opening time? The Hertz website seems to indicate that "after hours" return is available.
I don't have an answer to your question, but I do have some advice.
Take date stamped photos, and video of the car when picking up and returning. With no human contact, you will want to keep proof for several months after your return that you did not cause any damage to the car.
Adding to the good advices above:
Take the offered insurance without any deduct.