I assume that with all the skiers there must be some areas and times when fat bikes are not allowed.
Fortunately there are no fatbike bans, but you are not allowed to be an asshole. In other terms: ride with caution by yielding to skiers, passing at low speed, keep out of the ski track when riding prepared trails and be a nice guy.
If you are around the alpine ski centers like Grefsenkollen and Tryvann, I think there is a ban on bikes, allthough it is not communicated.
The best riding is anyway done on singletrack where hikers are frequent.