Oyo Empire metropolitan Oyo. This may have been fine in the 15th century when Oyo controlled only its heartland. But to make and maintain farther conquest, the structure underwent several changes. Oyo maintained a semi-standing army of specialist cavalry soldiers called the "Eso", the "Esho" or, formally, the Eso of Ikoyi. These were 70 junior war chiefs who were nominated by the Oyo Mesi and confirmed by the Alaafin of Oyo. The Eso were appointed for their military skill without regard to heritage and were led by the Are-Ona-Kakanfo. After Oyo's return from exile, the post of Are-Ona-Kakanfo was established as the
Yoruba people century. It is due to the influence of Bishop Samuel Ajayi Crowther, the first Anglican bishop in Nigeria. Crowther was himself an Oyo Yoruba and compiled the first Yoruba dictionary as well as introducing a standard for Yoruba orthography. The alternative name "Akú", apparently an exonym derived from the first words of Yoruba greetings (such as "Ẹ kú àárọ?" "good morning", "Ẹ kú alẹ?" "good evening") has survived in certain parts of their diaspora as a self-descriptive, especially in Sierra Leone. The Yoruba culture was originally an oral tradition, and the majority of Yoruba people are native speakers of the