Go with Norwegian Air. Last fall, we took several flights with them, within Norway. The prices and schedules were very good, but do note that they offer a variety of prices, depending on how much luggage you have, whether you want to reserve a particular seat, etc. Their App is easy to use.
One other thing: this may be all about the Oslo airport and not about Norwegian Air, but we arrived in plenty of time for our flight, and went to the gate for the flight. At one point, I just happened to double-check a flight monitor display, and the gate had been changed to another one, quite a distance away! There had been no announcement or gate-change sign, even at the original gate where we’d been waiting. If I hadn’t caught the different gate on the screen at the airport, we’d have missed our flight. Check the monitor early and often at Oslo, to avoid getting stranded.
Dogsled rides are another attraction at Tromsø, besides Northern Lights. Have a great birthday trip!