Here I think you are confusing the Oriental Orthodox and the Assyrian/Ancient Church of the East with the Eastern Orthodox, although the Oriental Orthodox had their own ecumenical councils, and their doctrine is extremely close to Eastern Orthodoxy, indeed, the Oriental Orthodox St. Severus of Antioch had an enormous influence on Eastern Orthodox theology by stressing theopaschitism and communicatio idiomatum, and his hymn Ho Monogenes is included in every Eastern Orthodox divine liturgy. In antiquity the lines between these churches were often blurred, a point further stressed by the scholarship of Sebastian Brock showing that the much-loved monastic theologian St. Isaac the Syrian was in fact a member of the Church of the East.
Also in the 19th century the Coptic Orthodox and Greek Orthodox churches of Alexandria attempted to unite, which would have at the time also included what are now the Ethiopian and Eritrean Orthodox (unless they broke away in a schism, which is possible, and that could have been disastrous for Ethiopia), but the Albanian Muslim Khedive, the ruler of Egypt who was nominally a liegeman of the Ottoman Sultan, but in reality enjoyed sovereignty and de facto independence from the Sublime Porte. However, now we have a new ecumenical arrangement, since around the year 2000, between the two Alexandrian Orthodox churches, which has many of the benefits of a full merger, without the risks or logistics headaches.
Unfortunately, a vocal minority of Eastern Orthodox are extremely hostile to the Oriental Orthodox, and on the flip side, some Ethiopian monks reportedly continue to regard the Eastern Orthodox as Nestorian.
The Church of the East has problems, like the continued schism between the Assyrian Church of the East and the much smaller Ancient Church of the East, but it does agree with the two Orthodox communions on several issues. For example, it rejects the Filioque and follows the same model of Apostolic Succession and Episcopal Polity (since 1974, when the last of the uncanonical hereditary Patriarchs of the Assyrian Church of the East was unfortunately the victim of an assassination, like so many others around that time; while I disagree with mich of what Mar Shimun XXIII Eshai, memory eternal, did, and proposed to do, his assasination was a tragedy, and probably resulted from his desire to get married, which ordinarily I would oppose for an Orthodox bishop, however, in the case of Mar Shimun XXIII, he never asked to be the Catholicos of the East, but inherited it from his uncle (the younger son of the older brother of each Catholicos would become the new Catholicos). This arrangement was a violation both of the ancient canons shared by all of the ancient churches, whether Orthodox, Catholic or Assyrian, as well as the specific canons of the Church of the East, and this led to the schism when Mar Shimun XXIII introduced the Gregorian Calendar, which was enough for the Indian bishop who discovered the canonical impropriety and other traditionalists to break away, crowning Mar Addai II as Catholicos of the Ancient Church of the East.*
*The title Catholicos is a title used by several presiding bishops of Asian churches who were at one time vice-Patriarchs to the Patriarch of Antioch, and in many churches such as Georgia and the Church of the East is used interchangeably with Patriarch. In the Syriac Orthodox Patriarch of Antioch, it was replaced with Maphrian, so that the vice-Patriarch in charge of the church in modern day Iraq would not be confused with his counterpart from the Church of the East. The independent, many feel schismatic, Malankara Orthodox Syrian Church in India revived the title for its presiding bishop. Finally, the two worldwide Armenian communions are the Catholicosate of Holy Etchmiadzin and All Armenia, and the Catholicosate of the Great House of Cilicia, dating back to the time when the Armenians ruled the Kingdom of Cilicia in addition to the Kingdom of Armenia. The latter received renewed relevance after the Soviet Union invaded and annexed Armenia in the 1920s, with many anti-Soviet Armenians joiming new parishes set up under Cilicia, but the two churches were reconciled adter the downfall of the Soviet Union. The two overlapping hierarchies persist as a legacy, but are no longer in schism with each other. There are also two independent Patriarchates, the Armenian Patriarchate of Jerusalem and the Armenian Patriarchate of Constantinople, which exclusively operate in their canonical territory and are respected by both Catholicoi.