Ethiopia till 1887 consisted of the northern highlands, thats all.
The region was and still is multiethnic and multireligous, there is still debate over
whether christianity is the majority or Islam they are probably close in number.
Oromos did not arrive in the region they were always there, so were dozens of different states and ethnic groups.
Between Ethiopia and the Oromos and Somalis were at least 6 major city states that were advanced for the region and highly urbanized and cultured,
The majority were muslim the largest being Ifat and later on Adal.
The Oromo nation was to the south west of Adal and was bordered on the south by the somalis and in the north by the various muslim states.
Back to question in the thread " Why did Ethiopia not expand south".
The short answer is yes is did, since Ethiopia historically was the northern highlands its expansion south and conquest of the various states between it and Adal and the Oromos and the Somalis was a major expansion.
For example Shoa which is today the heartland of the Amhara and the heartland of christian Ethiopia was a muslim sultanate when it was conquered by the Zagwe who evolved into the amhara.
This started a 300 year period of continuous warfare between muslims and Christians that lasted till the campaigns of Ahmed Gurey or Gran as the Ethiopians called him in the 1522-1540.
Till Ahmed gurey the Ethiopians were in constant expansion south and gobbled up most of the muslim states except Adal.
Adal survived and was able to counterattack and attempt reconquest northwards due to the manpower provided by Somalis.
Somalis were brought in similiar to how the Turks were brought into the Abbasid caliphate and used as soldiers.
Somalis assimilated into Adali culture and even joined the nobility,
one of such products Was ahmed gurey who rose from the ranks in the army and launched a coup d'etate in 1520 taking over regency of the Sultanate and launching a jihad that devastated Ethiopia for centuries after.
The Oromo nation were at this time neutral and were perfecting their unique and brilliant political, social and millitary system called gadaa.
Millitarizing their entire society and placing everyone in age grade millitary political units.
After a failed offensive southwards against the Somali Ajuran Sultanate,
they assimilated cavalry tactics bred horses and improved their tactics.
It was fortunate that the timing was perfect for their expansion, as the christians and muslims exhausted themselves in the 300 year war.
The Oromos moved north and destroyed every opposition with ease.
They also assimilated forcefully most of the former states and dozens of ethnic groups that are today thoroughly oromo.