So during the early 1990's, known as the TGE [transitional government of Ethiopia,] when they were debating federalism and the borders of the various states, the Oromo zone was established because they did not all agree that it belonged in Oromia proper. It's basically an Oromo island, within an Amhara ocean, [it's cut off from the rest of Oromia proper and is surrounded on one side by Amhara's, and on the other side by Afars]. They are, of course, by far, Muslims, so this adds to the so called ''threat'' that Amhara elites and their cadres feel that the Oromo there pose.
They are among the bravest Oromo's, and I think it's because them being surrounded, forced them to be even tougher, and they regularly fight the Amhara forces and repel their attacks, even while being out-numbered. The Amhara are now trying to recruit them to go fight in Tigray.
Yeah Wollo is part of it: this is the Oromia Special Zone in Amhara state, which has parts of Wallo-bAATI, Dawe, Esseya Gulla and Artuma. Kamisee/Kemise is the administrative center of this zone. It's pretty much an Oromo island in an Amhara sea/ocean.
The last census put the population at just under half a million in this zone, most of which are of course, Oromo.