My own theory is that the split between Oromo and Somali happened when Camels were introduced to Somalia about 1500 years ago. Oromo citizenship under Waaqafeena requires ownership of cattle, if you do not own cattle you are not Oromo, you are Gabbaro (or in Somali, Gaboye), essentially a subservient position.
Cave paintings show that early Pouni people worshipped cattle, so they took cattle ownership very seriously. Cattle herding is marginal in many parts of the Horn of Africa today though this was not always the case. Here is the horn of Africa today:

5000-7000 years ago though things were very different:

The slow drying of the horn drove cattle herders further south into the recent homeland of the Oromo people in Bale and north Kenya, but for religious and cultural reasons were slow to adopt camels, UNTIL Islam arrived. Essentially, a branch of the Pouni people near Harar scratching out a living herding their cattle became Muslims. This had a dramatic effect for two reasons, one: Islamic citizenship is not tied to cattle ownership, and two: Arabian influences encouraged camel use rather than discouraging it. Prior to this event, the only inhabitants of the semi-desert scrublands were small groups of hunters and gatherers, a branch of the Pouni people similar to the Waata people of today. The camel-herding Pouni expanded rapidly throughout Somalia and pushed aside the Waata-like Pouni from the best land. The defeated hunters became the Midgan people, and the camel herders became the Somali people.