This is a proper noun. Given your context (fortresses/defense), my guess is that this is a reference to Østenfjeldske forsvarskomite.

See chapter 2 in the index over Norges forsvarshistorie, bind 2 in the first reference I have enclosed. You can also find a brief reference to it on the Wikipedia entry for Fossumstrøkets festning, and in some Spydeberg municipality development documents. I cannot find many references online, and Store Norske Leksikon has nothing about this group, that seems to have been tasked with establishing the eastern defense lines against Sweden after the end of the Union in 1905. Thus it is part of the earliest defense history of modern Norway.