Orthodox are two different Communions of Apostolic Churches that separated from the Catholic Church in two different separations. First in 451, the Oriental Orthodox - consisting of the Armenian, Indian, Syriac, Ethiopian, Eritrean, and Coptic Churches - following the theology of St. Cyril of Alexandria regarding “miaphysitism” (not monophysitism as is often wrongly claimed by others). Miaphysitism is a legitimate position within the Catholic Church.
Later in 1054 - an estimate date - the Eastern Orthodox Churches - consisting of the Russian, Greek, Ukrainian, etc) separated from Rome due to political conflicts, the Crusades, killing of Latins in Constantinople, and the mutual excommunications of the Ecumenical Patriarch and a Cardinal representing a then-recently deceased Pope of Rome.
The theologies of both the Oriental and Eastern Orthodox are wholly acceptable to the Catholic Church. Both Communions have had groups reunited with the Latin Catholic Church in various times, making up the Eastern Catholic Churches. The Eastern CATHOLIC Churches follow the Liturgy, theology, Patrimony, calendar, Apostolic heritage, etc of their Orthodox counterparts, but are fully united to each other and the Pope of Rome.
It should be mentioned that there was an earlier separation before the two Orthodox Communions with Rome. The Church of Babylon, called the Assyrian Church of the East. This church separated in 431, when the Church accepted the Christology of Nestorius - then the Patriarch of Constantinople, who’s later followers theology was repudiated by St. Cyril of Alexandria, among others. The majority of this Church reunited with Rome and the Eastern Catholic Churches in the 1500s, making up the Chaldean Catholic Church. There remains a separation with those that did not unite, but the Christological controversy has been resolved.