UKH

Someone will know this....

Heading to Jotunheim in May...

Flying in to Oslo...

Where can I pick up standard stove gas cannisters ....?

Barbeg

Are you flying into Gardermoen?

I researched Jessheim for a trip once, as the town nearest the airport, and had found this place: https://www.jaktia.no/gass-og-bensin

In the end, we bought it across the border in Sweden as it was cheaper, but that doesn't help you. 
I've found that in the smaller towns in Scandinavia, a lot of sporting goods shops also have outdoor products, including gas.

Try XXL Oslo Sentrum - it's a 3 floor outdoor store right in the town centre and close to the station.

But going the wrong direction from Gardemoen..... and which station ?

Basically try any sports shop, with XXL being a large chain akin to Decathlon. Some garages as well, especially once you get away from town and towards Jotunheimen?  Where in Jotunheimen - so you know where you're going through.

https://www.xxl.no/store-finder

Beware all sports shops will be shut on a Sunday, except a very small number attached to cafes...

Maybe it is the wrong direction if you are renting a car. But not if you are taking public transport, when you have to go into town from the airport to pick up most long distance buses or trains

Biltema. You'll go past at least one on the way.

You'll find gas everywhere. XXL, Intersport, Anton Sport, Sport 1, Biltema. Take your pick. Every small town will have one of these somewhere. Some of the big DNT huts also sell gas I think

Went on a climbing trip in May this year the south / south west of Norway. We flew to Olso, hired a car and drove a few mins to Jessheim and picked up gas from a department store called Jula (I think they sold stoves too) which is right next to a big Rema 1000 for a big food shop if you need it. Not too far from the airport, easy parking and open early / late too. Hope this helps