In the "Encyrlical of the Eastern Patriarchs" of 1848, which was a reply to the epistle of Pope Pius IX, "To The Easterns," the Patriarchs of Constantinople, Alexandria, Antioch, and Jerusalem, along with the other assembled bishops stated: "Our Church holds the infallible and genuine deposit of the Holy Scriptures: of the Old Testament a true and perfect version, of the New the divine original itself." And so we have always held that the Septuagint is the authoritative version of the Old Testament.
...the basis of the Old Testament text in the Orthodox tradition is the Septuagint, a Greek translation by the "seventy interpreters" made in the third to second centuries BCE for the Alexandrian Hebrews and the Jewish diaspora. The authority of the Septuagint is based on three factors. First of all, though the Greek text is not the original language of the Old Testament books, the Septuagint does reflect the state of the original text as it would have been found in the third to second centuries BCE, while the current Hebrew text of the Bible, which is called the "Masoretic," was edited up until the eighth century CE. Second, some of the citations taken from the Old Testament and found in the New mainly use the Septuagint text. Third, the Septuagint was used by both the Greek Fathers of the Church, and Orthodox liturgical services (in other words, this text became part of the Orthodox church Tradition). Taking into account the three factors enumerated above, St. Philaret of Moscow considers it possible to maintain that "in the Orthodox teaching of Holy Scripture it is necessary to attribute a dogmatic merit to the Translation of the Seventy, in some cases placing it on equal level with the original and even elevating it above the Hebrew text, as is generally accepted in the most recent editions (Orthodox Christianity, Volume II: Doctrine and Teaching of the Orthodox Church, (New York: St. Vladimir Seminary Press, 2012) p. 34).
There was a time when many Protestant scholars assumed that the Septuagint was an often loose translation of the Hebrew text, and that when it differed from the Masoretic Text, it was due to changes made by the translators. However, since the discovery of the Dead Sea Scrolls, we now know that the Septuagint is based on a different, and older Hebrew text than the Masoretic text.
The Hebrew Text that has served as the basis for most translations of the Old Testament into English is based almost entirely on the Leningrad Codex, which dates from 1008 A.D. In comparison to the textual evidence that we have for the New Testament Greek text, this is a very late manuscript. It is an example of the Masoretic recension, which is usually dated to have been shaped between the 6th and 10th centuries A.D. This is well after the Septuagint was translated (3rd century before Christ), the Peshitta (1st and 2nd Centuries A.D.), or the Latin Vulgate (4th Century A.D.). According to Christian tradition, the non-Christian Jews began making changes in the Old Testament text to undercut the Christian use of Old Testament prophecies concerning the coming of Christ. In any case, the Hebrew Text that we now have was preserved outside the Church. The Septuagint and Peshitta texts were preserved within the Church, and so the Church believes that the text of the Old Testament was been authoritatively preserved in these textual traditions.