Hello, sorry for the late reply! I just had to send the question to Andrei, an Orthodox theologian, because it was way too complicated for me!
So, here is his reply:
"There is an Ecumenical Council that both the Oriental (except, of course the Nestorian Church) and the Eastern Orthodox Churches accept: the 3rd Ecumenical Council, in 431, at Ephesus. This Council decided that Christ was (and is) fully God and fully Man (complete God and complete Man), thus having two natures. According to Wikipedia, this is the moment when the Assyrian Church of the East broke up with the Orthodox Church.
And then, there is another Ecumenical Council, which was not accepted by the Oriental Orthodox Church: the 4th Ecumenical Council, at Chalcedon, in 451. The decision (I think it was called Oros) says it very clear (my emphasis):
We, then, following the holy Fathers, all with one consent, teach people to confess one and the same Son, our Lord Jesus Christ, the same perfect in Godhead and also perfect in manhood; truly God and truly man, of a reasonable [rational] soul and body; consubstantial [co-essential] with the Father according to the Godhead, and consubstantial with us according to the Manhood; in all things like unto us, without sin; begotten before all ages of the Father according to the Godhead, and in these latter days, for us and for our salvation, born of the Virgin Mary, the Mother of God, according to the Manhood; one and the same Christ, Son, Lord, only begotten, to be acknowledged in two natures, inconfusedly, unchangeably, indivisibly, inseparably; (ἐν δύο φύσεσιν ἀσυγχύτως, ἀτρέπτως, ἀδιαιρέτως, ἀχωρίστως – in duabus naturis inconfuse, immutabiliter, indivise, inseparabiliter) the distinction of natures being by no means taken away by the union, but rather the property of each nature being preserved, and concurring in one Person (prosopon) and one Subsistence (hypostasis), not parted or divided into two persons, but one and the same Son, and only begotten God (μονογενῆ Θεὸν), the Word, the Lord Jesus Christ; as the prophets from the beginning [have declared] concerning Him, and the Lord Jesus Christ Himself has taught us, and the Creed of the holy Fathers has handed down to us.
The person who asked you said:
I feel like the solution given at the Council is valid however the Oriental Orthodox could be correct too.
There is a logical rule that states that A cannot be equal to B. So either the Council at Chalcedon was right, either the Oriental Church was right; and we know that the Council was right. The Oriental Church cannot be right too; only if the Oriental Church accepts the dogmatic definition of the Council, in which case it becomes Orthodox. So, if the Oriental Church is not Nestorian or Monophysite, and accepts the dogmatic definition at Chalcedon, which, by the way, belongs to Saint Cyril of Alexandria, then the Church is hereby Oriental Orthodox; there is one example of such a Church, and that is the Coptic Egyptian Church, with whom we share the same dogmas.
The Council of Chalcedon have thrown the anathema on all those who do not accept its definition. Also did the Council at Ephesus, and the other 5 Ecumenical Councils. So, if anybody still believes that Christ had only one nature, either divine or human, they are either Nestorians or Monophysite, and they go under the action of the anathemas. If the same person agrees the solution at Chalcedon, then very good for him/her.
I hope the answer is clear.”
God bless you!