Verified answer

Pretty much every damn one, miss. It is as common as donuts are in the States or Fish&Chips in the UK.

But for you, I did a quick online search with my incredible Norwegian prowess. There are 158 licensed bakeries carrying skolebrød in the Oslo area. So really you could just walk in any direction and find one. Or, alternatively, put "bakery" into Google Maps' "search nearby" function after searching for a place in Oslo and print out the map you get.

PS: Do note that due to the commonality of skolebrød, some high-end bakeries might not carry such a "plebeian" and relatively tasteless cake.

Every single bakery in norway sells it....(many grocery stores too)