Hararghe highlands used to be dominated by Somalis specifically by the Dir tribe. It was ruled by the Adal Sultanate. The reason why the Oromos were able to penetrate into that region is because Adal Sultanate was weakened after engaging wars with Abyssinia and the Portuguese. After the Oromos occupied that area, many Somali clans were assimilated.
Oromos are newcomers to that area.
In fact, Dawaro Sultanate was another Somali kingdom centred around Hararghe.
Dawaro at one point were a vessel state of Adal Sultanate under Ahmed Gurey rule until he died and that was the beginning of the Oromo expansion.
Here is an interesting quote on the assimilation against the Jarso clan.
"By Somali reckoning, the founder of Dawaro Sultanate was Jārso people belonging to the Ali Madaḥweyne branch which is a sub-clan of Dir. After the Oromo migrations, the ʿAli Madaḥweyne Dir were absorbed into the Afran Qallo Barentuma confederation, and today the Jarso are reckoned as Jārsō Daggā Qāllō Barentuma Oromō."
Source: Being and Becoming Oromo: Historical and Anthropological Enquiries page. 233.