The term Western Orthodoxy is sometimes used to denominate what is technically a Vicariate within the Antiochian Orthodox Church and thus a part of the Eastern Orthodox Church as that term is defined here. The term "Western Orthodox Church" is disfavored by members of that Vicariate.
In the 5th century, Oriental Orthodoxy separated from Chalcedonian Christianity (and is therefore separate from both the Eastern Orthodox and Roman Catholic churches), well before the 11th century Great Schism. It should not be confused with Eastern Orthodoxy.
There is no single earthly head of all the Orthodox Churches comparable to the Pope of Rome. The highest-ranking bishop of the communion is the Patriarch of Constantinople, who is also primate of one of the autocephalous churches. These organizations are in full communion with each other, so any priest of any of those churches may lawfully minister to any member of any of them, and no member of any is excluded from any form of worship in any of the others, including reception of the Eucharist. Each local or national Orthodox Church is a portion of the Orthodox Church as a whole.
In the early Middle Ages, the One Holy Catholic and Apostolic Church was ruled by five patriarchs: the bishops of Rome, Constantinople, Alexandria, Antioch, and Jerusalem; these were collectively referred to as the Pentarchy. Each patriarch had jurisdiction over bishops in a specified geographic region. This continued until 927, when the autonomous Bulgarian Archbishopric became the first newly-promoted patriarchate to join the additional five.
The patriarch of Rome was "first in place of honor" among the five patriarchs. Disagreement about the limits of his authority was one of the causes of the Great Schism, conventionally dated to the year 1054, which split the church into the Roman Catholic Church in the West, headed by the Bishop of Rome, and the Eastern Orthodox Church, led by the four eastern patriarchs. After the schism this honorary primacy shifted to the Patriarch of Constantinople, who had previously been accorded the second-place rank at the First Council of Constantinople.
The four ancient patriarchates are most senior, followed by the five junior patriarchates. Autocephalous churches whose leaders are archbishops follow the patriarchates in seniority, with the Church of Cyprus being the only ancient one (AD 434). From the Orthodox point of view there would be five ancient patriarchates had the Great Schism not occurred, severing the Church of Rome from the Orthodox Churches in the 11th century.
Due in part to the re-establishment of official ties between the Russian Orthodox Church Outside Russia and the Moscow Patriarchate, the Orthodox Church of Greece (Holy Synod in Resistance) has broken ecclesial communion with ROCOR, but the converse has not happened. Where the Old Calendar Romanian and Bulgarian churches stand on the matter is as yet unclear.
The following Churches recognize all other mainstream Orthodox Churches, but are not recognized by any of them due to various disputes:
The Russian Orthodox Church in America holds a policy much like the Churches list above as 'In Resistance'. Communing the faithful but not concelebrating among hierarchs. The ROCIA's status is unclear, with many faithful and even priests received into other Orthodox Churches including ROCOR, the GOA and the OCA with their sacraments recognized, but as the Hierarchs of the ROCIA do not seek to concelebrate with other Churches, the exact standing of those hierarchs remains unclear.