but for St. Cyril the miaphysite formula was in harmony with the duaphysite formula, which the OO refuse to confess -- same words, different understanding
Alas, no. St. Cyril described our Lord as From two natures. St. Cyril also achieved a reconciliation with Patriarch John of Antioch. Both of these are accepted by the Oriental Orthodox.
What they reject is the Tome of Leo, which does appear at first glance to disagree with St. Cyril. There was an outbreak of crypto-Nestorianism following Chalcedon, albeit not in what is now the Eastern Orthodox Church; Chalcedon correctly anathematized Eutyches, as did the OO, but the poltiics became ugly. The crypto-Nestorianism ended with the Fifth Ecumenical Council, which in turn caused a brief schism in the Roman Church: the Three Chapters Controversy, which was in a sense the last hurrah of the crypto-Nestorians.
Unfortunately, by the time of Justinian, a state of unease existed between the EO and OO, and what wound up happening was an ethnic divide: Eastern Orthodoxy wound up with the Greek speaking halves of the Antiochian and Alexandrian patriarchates, whereas the Syriac and Coptic portions became Oriental Orthodox. The Ethiopians wound up under the omophorion of the Coptic Pope, and the Armenians were Oriental Orthodox, and for a brief time, there were pro-OO elements in the Georgian church.
So, ultimately, it became an ethnic divide, with "bad blood" existing on ethnic lines. The Chalcedonian Syrians I think were the last hold out, but with the rise of the Arabic vernacular, they wound uo becoming fully Byzantinized liturgically by the 13th century.
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There were for a time oddities in the Armenian church, and an Armenian-Syriac Orthodox schism. I expect this was related to the controversy involving the Armenian use of unleavened bread, and an unmixed chalice; all other OO churches use leavened prosphora closely resembling that used by the EO, with similiar preparation rites. This was resolved, fortunately. I believe unleavened bread is not inherently unorthodox, and if we do reunite with Rome, which I hope for, I think the Armenian-OO reconciliation provides a model for doing it (I believe our first priority should be reunion with the OO, followed by reunion with Rome and the de-Nestorianization of the Assyrian Church).
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In summary, at present, my view is that the OO are essentially Orthodox; the theological dispute separating us has been irrelevant since Justinian, but the schism was artificially prolongued by certain persecutions, bad blood, and the Oriental church becoming the church of ethnic Copts and Syriacs, and the Eastern church becoming the church of ethnic Alexandrian Greeks and Arabs (with the exception of some Syriac speaking communities in Syria which are Antiochian and presumably have Arabic or Greek liturgy, including the victims of persecution in Mallala).
As a ray of hope, consider also St. Isaac the Syrian: an Assyrian monk of the Church of the East, venerated by the OO and EO.
Also, I know Oriental Orthodox who venerate St. John of Damascus despite his criticism of them (which in fact primarily applies to tritheists, and to John Philoponus, who the Copts regard as a heretic). A Coptic parish I have visited has laminated cards in every row containing the pre-communion prayers of the Damascene saint in English and Arabic.
I have read reports of a ROCOR priest in Canada who served the Eucharist to Copts, oddly enough, given ROCOR's anti-ecumenism, a limited intercommuniom without concelebration exists betweem the Greek amd Syriac Patriarchates of Antioch in the Middle East, but not so much in the US.
So clearly, we are moving closer to ecumenical reconciliation. We are still a ways off from full Eucharistic communion, but as Metropolitan Kallistos Ware and others have pointed out, the main apparent theological barriers have been, by and large, overcome.
Most of the people I encounter ardently opposed to ecumenical reconciliation, by the way, are either converts or old calendarists. There remains some opposition among the Athonite monks, who I respect, however, recently, some Coptic monks were warmly welcomed there.