Empty your water bottle before going through security checkpoints. There are drinkable water faucets behind of the security checkpoints, nearby DNB currency withdrawal machines. Refill bottles there.
If you are a student of an institution that uses an eduroam hotspot, you may be able to get some signal here. That way you don't need to log-on to other hotspots.
For a city like Oslo, Flytoget (airportexpress train) isn't priced too badly. It's cheaper than Stockholm's Arlanda Express at only NOK340 (£38) vs SEK500 (£50) return. Half-price for students.
Waiting for a domestic flight you'll find power outlets under the high bench along the wall at Monolitten and along the bottom by the windows at the bar on the left halfway down.
If you struggle with your Staralliance Gold card for fast track, ask officer to try on different scanners..Worked well for me with Aegean Gold on SAS flight.
If possible use the self service check-in by the entrance. You'll get your boarding card & luggage tag, just drop your luggage at the luggage drop point. It's easier & faster than ordinary check in!
Take the Nor-Way bus express to anywhere in Oslo. Departure every 20 minutes, clean and comfortable coaches. Unless your destination is near a railway station, it's cheaper and quicker than the train.
When leaving on an international flight, walk to the left to avoid the horrible, perfume smelling, full of bright lights and nagging advertisements and cosmetics pushing tax free. Aaaah, serenity now.
Long layover? The bathrooms down the stairs next to the fine salmon shop have bigger vestibules with private sinks in each for cleaning up. Kinda hidden but such a good find.
If you are flying further with SAS or Norwegian, don't forget to collect your luggage! It's obligatory for all international transit/transfer passengers.