If it's -17 you've still got 0.8 degrees to work with before hitting 0F.
(That said, in reality, from my experience Blindern seems to be a reasonably warm site, probably because the instruments are on a bit of a mound, and -17 there probably means below -20 in valleys away from the central city).
Yeah, these days it's not uncommon to see colder temps at both Tryvann and Bygdøy. Of course, by the time we get down to the area around -17 it's all quite cold and really just needs to be a bit warmer. I like -5. -5 is a good winter temperature.