In the mid-sixteenth century, its political and military organization already weakened by the Muslim assault, the Christian kingdom began to be pressured on the south and southeast by movements of the Oromo (called Galla by the Amhara). These migrations also affected the Sidama, Muslim pastoralists in the lowlands, and Adal. At this time, the Oromo, settled in far southern Ethiopia, were an egalitarian pastoral people divided into a number of competing segments or groups but sharing a type of age-set system (see Glossary) of social organization called the gada system (see Glossary), which was ideally suited for warfare. Their predilection toward warfare, apparently combined with an expanding population of both people and cattle, led to a long-term predatory expansion at the expense of their neighbors after about 1550. Unlike the highland Christians or on occasion the lowland Muslims, the Oromo were not concerned with establishing an empire or imposing a religious system. In a series of massive but uncoordinated movements during the second half of the sixteenth century, they penetrated much of the southern and northern highlands as well as the lowlands to the east, affecting Christians and Muslims equally.

These migrations also profoundly affected the Oromo. Disunited in the extreme, they attacked and raided each other as readily as neighboring peoples in their quest for new land and pastures. As they moved farther from their homeland and encountered new physical and human environments, entire segments of the Oromo population adapted by changing their mode of economic life, their political and social organization, and their religious adherence. Many mixed with the Amhara (particularly in Shewa), became Christians, and eventually obtained a share in governing the kingdom. In some cases, royal family members came from the union of Amhara and Oromo elements. In other cases, Oromo, without losing their identity, became part of the nobility. But no matter how much they changed, Oromo groups generally retained their language and sense of local identity. So differentiated and dispersed had they become, however, that few foreign observers recognized the Oromo as a distinct people until the twentieth century.

In a more immediate sense, the Oromo migration resulted in a weakening of both Christian and Muslim power and drove a wedge between the two faiths along the eastern edge of the highlands. In the Christian kingdom, Oromo groups infiltrated large areas in the east and south, with large numbers settling in Shewa and adjacent parts of the central highlands. Others penetrated as far north as eastern Tigray. The effect of the Oromo migrations was to leave the Ethiopian state fragmented and much reduced in size, with an alien population in its midst. Thereafter, the Oromo played a major role in the internal dynamics of Ethiopia, both assimilating and being assimilated as they were slowly incorporated into the Christian kingdom. In the south, the Sidama fiercely resisted the Oromo, but, as in the central and northern highlands, they were compelled to yield at least some territory. In the east, the Oromo swept up to and even beyond Harer, dealing a devastating blow to what remained of Adal and contributing in a major way to its decline.

Data as of 1991