What are you on about, you speak as if the xarla are an ethnicity all on their own with no tangible evidence what so ever. Xarla also lived in the border of modern day somalia alongside their somali brethren. Even if we were to suppose for argument sake that they were a stand alone clan there are no actual evidence that lead us to believe that they proceeded somalis in hararghe all we have is but an account of their individual clan name that's all. Either way oromos themselves have only been their for about 500 hundred years while the area was inhabited for at least more than a thousand plus years meaning oromos have no stake in hararghe in terms of it's history beyond a certain point.
Oromo in Hararge have always been there. Cultural and language shift occurred and many assimilated into Oromo. Be it east or west Hararge. Many of the Ittu and Karrayu Oromo(Eastern Shawa) are of Harla origin. And the AQ are mixed of Harla and Somali stock. There is no proof of Harla being of Somali origin. Unless there is pure fact of this, we can only assume and for now, they were their own people. Many clans of Oromos have oral history about the Harla and still practice many of their old ways of like with herbal medicines etc.