Somalis and Oromos have a long, long history together. One of Hawiye's wives Giireey was an Oromo and Hawiye probably lived around the year 800 or so, his other wife was a Harari.
The current story about Oromos arriving in the 16th century is rubbish, Oromos, Somalis and Hararis have all been in contact with each other for at least 1200 years.
I think the belief about Oromos arriving in the 16th century is that the two confederations (Barentu and Borana) were originally in the south, and then moved north and absorbed everyone who is today part of the Oromo nation, and that the vast majority of Oromo clans have been re-organized into the two Oromo confederations and their original origins are either now obscure or confused. We know that for instance Hawiye is organized into the Barentu confederation, despite the fact that we know that Hawiye are not Oromo and predate the Barentu confederation by many centuries.
It's essentially the concept of Sheegaad (client/patron protection) on a massive scale. Rendiile do the same thing, so I guess originally, before Islam, this was the way the lowlanders of the Horn organized themselves politically, and survives within the Sheegaad system, even though it has largely been replaced amongst the Somalis by the Arab lineage-based clan system.
Strangely the confederation system still exists amongst the Afar, though I believe that the Afar became Muslims after the Somali did.
While rather controversial to talk about, the major clans in Somalia may have began as confederations before gradually changing to the Arab style clan system, though that's basically an "argument in a can", as soon as you open it, expect arguments
