What are the differences between Eastern Orthodoxy, Oriental Orthodoxy, and Nestorianism?

There is indeed a difference ,sadly. The Eastern believe in two wills of the incarnated Son, while the Oriental believe in one will, and that (if you are asking me) is greatly a reason for schism, that means that either the Oriental have the "Fully Human" definition wrong, or that Eastern got it wrong ,and for either cases there must be someone who is wrong and someone who is right!

I am an Oriental Orthodox Christian myself but am freethinking. By this I mean I do not follow my faith blindly or like a robot; but rather study it to the T and compare it to that of the Eastern. Before I go on I would like to state first and foremost that in today's world, there is absolutely no reason for the two main branches of Orthodoxy (Eastern and Oriental) to be separated; and I pray every single day that our unity and communion may come to fruition. May the Lord grant this to us, my brothers and sisters in Christ.

If you will hear me I've sort of laid down the Oriental Orthodox belief in Christ's Nature(s). This in no way attacks the Eastern Orthodox theology of Christ, as that is never my intention! My only intention is to share what is widely misconceptualized throughout the Christian world.

One important thing to note is that the Oriental Orthodox Church does indeed take notice of the two natures of Christ. These natures being His fully human nature as well as His fully divine nature. However, they believe these 2 natures come into One unique nature dubbed (as St.Cyril himself put it) "The One Nature of God The Incarnate Logos."

Something very important to note and is widely unnoticed is the confession by which Oriental Orthodox Christians make before communion. The Priest or Bishop praying the respective liturgy states, "He made it (His flesh) one with His divinity without mingling, without confusion, without alteration." Thus, showing how Oriental Orthodox Christians do not entirely reject the 2 natures proposal, but rather the contrary! They believe these 2 natures come together into 1 nature without mixing, and without separation! To them, 2 natures hints at separation, which is the rejection they hold. This is also shown in the same confession they make before Eucharist as it states, "Truly I believe His divinity parted not from His humanity for a single moment nor a twinkling of an eye." Quite similar to the way The Eastern Orthodox Church believes that the 2 natures of Christ were not seperated as well! A unique, unmixed, inexplicable in human terminology, One Nature of God The Incarnate Logos.

As I've stated prior, I as well as many Orthodox Bishops and Priests from both branches of Orthodoxy do not see any reason for such a split.

In fact, in some parts of the Middle East where Christians are persecuted to an immense degree, Eastern as well as Oriental Orthodox Bishops have come to a decree by which members may intermarry and take communion at either church! (Some people may not know that we also recognize each other's baptisms.) The only catch is that whichever church holds said marriage (Eastern or Oriental), must have their respective priests and bishops be responsible for any (God forbid) counseling to the married couple should it be needed.

This just goes to show how minute the differences are and how in a sense it's more political than theological in terms of reasoning for as to why the churches aren't in communion!

Lastly, in the 1990s or so both branches were going to join in communion! To add to this, every church had signed the document agreeing to such! However, for a certain reason (which I choose not to mention due to any bashing to either side that I pray may not occur from this post) it did not come to be. I will say however, the reason was political.

I think our jobs as Orthodox Christians from either branch in regards to this matter, is to pray for the union! To all my Eastern Orthodox Christian brothers and sisters reading this, though I may not know you, I love you! And I would love to one day in my lifetime here on Earth, be able to as an Oriental Orthodox Christian partake of the only holy body and precious blood, of our Lord God and Savior Jesus Christ, with you all.

As pointed out in that question, the differences were originally in Christology. Generally the fourth-century Christological debates were about how the relationship between the nature(s) of Christ and the person of Christ.

The "Nestorian" party held that there were two natures in Christ, the divine and the human, with very little relationship between them. Essentially there were two hypostases ("persons"), the human and the divine.

The "monophysite" party (which became the Oriental Orthodox) held that there was one nature in Jesus Christ, in which the human and the divine were brought together. Monophysitism was essentially a reaction against Nestorianism.

The remaining party is best known as the "Chalcedonian" party after the Council of Chalcedon: the Eastern Orthodox and all Western churches derive from this party. It held a view between the two: there were two natures in Christ and one person. The famous definition is the Chalcedonian definition:

in Two Natures unconfusedly, unchangeably, indivisibly, inseparably; the difference of the Natures being in no way removed because of the Union, but rather the properties of each Nature being preserved, and (both) concurring into One Person and One Hypostasis; not as though He were parted or divided into Two Persons, but One and the Self-same Son and Only-begotten God, Word, Lord, Jesus Christ

It is frequently argued that there is no substantial difference between the Chalcedonian and Monophysite positions. (It's worth noting, BTW, that the Oriental Orthodox prefer the term "Miaphysite", which essentially means the same but lacks the connotations of heresy.)

As to other theological and liturgical differences? Well, they have been divided for 1700 years or thereabouts and they grew up separately, often regarding the others as heretical. It should be no surprise, then, that they have substantial differences in liturgy and theology. All three traditions are very tradition-centred and give a special place in their theology to great theologians of the past. So the Eastern Orthodox will quote Maximus the Confessor, while the Oriental Orthodox might quote Gregory the Illuminator. But again, it's not so simple: the Armenian Orthodox (an Oriental Orthodox church) are very different from the Coptic Church (another Oriental Orthodox church) and are not in communion with one another.

The differences are complex and very old: too many to mention in this context. But if I may give an opinion as an ecumenist: the Christological differences started the divisions and dominate in ecumenical dialogue between the churches, yet they are not the reason for the modern-day division. The Oriental Orthodox and the Eastern Orthodox agreed a common statement on Christology a few years ago, but they are not in full communion because the cultural and theological divisions are too great to bridge in so short a period of time.