The Oriental Orthodox churches are distinguished by their recognition of only the first three ecumenical councils during the period of the State church of the Roman Empire — the First Council of Nicaea in 325, the First Council of Constantinople in 381 and the Council of Ephesus in 431. Oriental Orthodoxy shares much theology and many ecclesiastical traditions with the Eastern Orthodox Church; these include a similar doctrine of salvation and a tradition of collegiality between bishops, as well as reverence of the Theotokos and use of the Nicene Creed.
The primary theological difference between the two communions is the differing Christology. Oriental Orthodoxy rejects the Chalcedonian Definition, and instead adopts the Miaphysite formula, believing that the human and divine natures of Christ are united. Historically, the early prelates of the Oriental Orthodox churches thought that the Chalcedonian Definition implied a possible repudiation of the Trinity or a concession to Nestorianism.
Other differences include minor deviations in social teaching and different views on ecumenism. Oriental Orthodox churches are generally considered to be more conservative with regard to social issues as well more enthusiastic about ecumenical relations with non-Orthodox churches.
The break in communion between the Imperial Roman and Oriental Orthodox churches did not occur suddenly, but rather gradually over 2-3 centuries following the Council of Chalcedon. Eventually the two communions developed separate institutions, and the Oriental Orthodox did not participate in any of the later ecumenical councils.
The Oriental Orthodox churches maintain their own ancient apostolic succession. The various churches are governed by holy synods, with a primus inter pares bishop serving as primate. The primates hold titles like patriarch, catholicos, and pope. Among these patriarchs, the Pope of Alexandria takes precedence, and is sometimes considered the "face" of Oriental Orthodoxy. The Alexandrian Patriarchate, along with Rome and Antioch, was one of the most prominent sees of the early Christian Church, and contains a majority population of Coptic Christians, and unlike Antioch is still a major population center.
That said, the Pope of Alexandria has no governing powers with respect to the non-Coptic churches. Oriental Orthodoxy does not have a magisterial leader like the Roman Catholic Church, nor does the communion have a leader who can convene ecumenical synods like the Eastern Orthodox Church.
The schism between Oriental Orthodoxy and the adherents of Chalcedonian Christianity was based on differences in Christology. The First Council of Nicaea, in 325, declared that Jesus Christ is God, that is to say, "consubstantial" with the Father. Later, the third ecumenical council, the Council of Ephesus, declared that Jesus Christ, though divine as well as human, is only one being, or person (hypostasis). Thus, the Council of Ephesus explicitly rejected Nestorianism, the Christological doctrine that Christ was two distinct beings, one divine (the Logos) and one human (Jesus), who happened to inhabit the same body. The churches that later became Oriental Orthodoxy were firmly anti-Nestorian, and therefore strongly supported the decisions made at Ephesus.
Twenty years after Ephesus, the Council of Chalcedon reaffirmed the view that Jesus Christ was a single person, but at the same time declared that this one person existed "in two complete natures", one human and one divine. Those who opposed Chalcedon saw this as a concession to Nestorianism, or even as a conspiracy to convert the Church to Nestorianism by stealth. As a result, over the following decades, they gradually separated from communion with those who accepted the Council of Chalcedon, and formed the body that is today called Oriental Orthodoxy.
At times, Chalcedonian Christians have referred to the Oriental Orthodox as being Monophysites–that is to say, accusing them of following the teachings of Eutyches (c. 380 – c. 456), who argued that Jesus Christ was not human at all, but only divine. Monophysitism was condemned as heretical alongside Nestorianism, and to accuse a church of being Monophysite is to accuse it of falling into the opposite extreme from Nestorianism. However, the Oriental Orthodox themselves reject this description as inaccurate, having officially condemned the teachings of both Nestorius and Eutyches. They define themselves as Miaphysite instead, holding that Christ has one nature, but this nature is both human and divine.
Today, the Oriental Orthodox churches are in full communion with each other, but not with the Eastern Orthodox Church or any other churches. Slow dialogue towards restoring communion between the two Orthodox groups began in the mid-20th century, and dialogue is also underway between Oriental Orthodoxy and the Catholic Church and others. In 2017, the mutual recognition of baptism was restored between the Coptic Orthodox Church of Alexandria and the Catholic Church. Also baptism is mutually recognized between the Armenian Apostolic Church and the Catholic Church.