Short answers, pulled from the story.
The Oriental Orthodox Churches are a communion of six autocephalous Eastern Christian churches adhering to Miaphysite Christology. Membership estimates range from approximately 57 million to 87 million, depending on whether external or self-reported figures are used, with the Ethiopian Orthodox Tewahedo Church being the largest of the six.
The split followed the Council of Chalcedon in 451, which declared Christ existed in two complete natures, one human and one divine. Oriental Orthodox hierarchs rejected this as too close to the condemned doctrine of Nestorianism, preferring Cyril of Alexandria's formula of "one nature of God the Incarnate Logos." The break in communion developed gradually over two to three centuries after Chalcedon.
Miaphysitism holds that Christ has one nature that is fully and inseparably both human and divine. Monophysitism, associated with Eutyches around 380, held that Christ was only divine and not human, a position condemned as heretical. The Oriental Orthodox reject the monophysite label and have officially condemned the teachings of both Nestorius and Eutyches.
Armenia has the highest share, with Oriental Orthodoxy accounting for 94% of the population. In Ethiopia, it accounts for 43.1% nationally, reaching 96% in the Tigray region and 75% in Addis Ababa. Eritrea is approximately 40% Oriental Orthodox.
The name "Oriental Orthodox Churches" was formally adopted at the Conference of Addis Ababa in 1965. At that time, five churches participated; the Eritrean Orthodox Tewahedo Church was not yet autocephalous and achieved that status in 1993.
Syriac patriarch Mar Ignatius Zakka I Iwas and Pope John Paul II issued a common statement in 1984 declaring that the historic schisms between their churches arose only from differences in terminology and culture, not from genuine doctrinal divergence on the Incarnation. They concluded there was "no real basis" for the divisions that followed the Council of Chalcedon.