The problem with Oromos is that they descend heavily from pre-existing populations of the Ethiopian highlands (Agaws in the Northwest and Omotics in the Southwest). The ones bordering Somalis are closer to the original Oromos but due to mixing with the other Oromos even those have been changed from their ancestral state.
They have a culture of assimilating conquered peoples and they also consider anyone who speaks Oromo as being Oromo. So that's why they easily absorbed many Somali clans in the past.