In this post I am going to attempt to explain why the Orthodox Church uses the Septuagint as the basis for its Scripture instead of the Masoretic text used in most Bible translations today.
First, what is the Septuagint? From Wikipedia:
"The Septuagint ( //), or simply "LXX", or the "Greek Old Testament", is a translation into Koine Greek of the Christian Old Testament. It incorporates the oldest of several ancient translations of the Old Testament, Biblical apocrypha and Deuterocanonical books. The LXX is referred to in critical works by the abbreviation  or G.
Septuagint was originally the designation for the Jewish Torah translated into Koine Greek, thelingua franca of the Eastern Mediterranean from the death of Alexander the Great (323 BCE) until the development of Byzantine Greek (c.600 CE). Some early pre-Christian Jewish versions of the Septuagint, were held in great respect in ancient times; Philo, the Hellenistic Jewishphilosopher, and Josephus, the 1st-century Romano-Jewish historian, ascribed divine inspiration to the Jewish translators."
The Septuagint was translated in the 3rd century BC. This was probably only the first five books - the Pentateuch (or the Torah) - but could have been the whole of the Hebrew Scriptures. Either way by about the first century BC the Greek translation was complete and used during Jesus' time.
An interesting thing to note is that the Jews actually celebrated the translation of the Septuagint since they mostly spoke Greek (and Aramaic); Hebrew had ceased to be spoken by the majority of Jews. Philo wrote in Greek and used the LXX in his writings. OrthodoxAnswers has this to say:
"The Septuagint translation was an occasion of great celebration, and a special day was set aside to commemorate this event in the Jewish community, which, for the most part, no longer spoke Hebrew, especially in the diaspora. (In Palestine the Jews spoke only Aramaic.) Now, with the Septuagint translation, the rabbis could instruct their people again easily in a language most of them spoke (Greek), but, in addition, they could make their faith more readily accessible to the pagan world around them. Consequently, the Septuagint was held in great esteem, and in the time of our Saviour, it was in wide use inthe Jewish community (as the many quotations from it in the New Testament testify). What is also noteworthy is that Philo, one of the greatest Jewish scholars of antiquity, was also one of the foremost apologists for the Jewish religion among the pagans. Through the many tracts he wrote (all of them based on the Septuagint text), he led many thousands of pagans to convert to the Jewish faith. Yet, Philo, a contemporary of our Saviour, could not speak Hebrew. He knew only Greek."
It seems that by the time of Christ that the Hebrew language was pretty much like the Latin of today; only the very learned used it and it was used mostly in a liturgical setting. It makes sense, therefore, that the Early Church would continue to use the Greek translation as Christianity spread throughout the Roman Empire (of which Greek was used extensively in the Mediterranean region). Also, it would be safe to assume that the Deuterocanonicals (aka Apocrypha) were used and included in the canon of the Old Testament before and after Jesus' lifetime - as I mentioned in my post In Defense of Icons Part 2 - evidence of which can be found by certain practices of the Jews in the first century. Wikipedia has this to say (note that the Hebrew canon mentioned in the below quote is in reference to changes made after Christianity had been widely spread):
"Some of the books not admitted into the Hebrew canon, such as the Wisdom of Solomon and 2 Maccabees (originally Greekcompositions) gave the only textual support for the common first century Jewish belief in the after-life. The martyrs' prayers for the dead and the living praying and offering sacrifices for the dead motivated Martin Luther to reject these books as apocryphal because they supported Catholic doctrine and practice."Naturally, the use of the Greek text to include the Deuterocanonicals carried over into early Christian practice. But why do the Jews of today not use the Greek text that was used back in those days? To save their language and their culture, perhaps?
With the destruction of Jerusalem in AD 70 and the expansion of Christianity it is possible that the Jewish culture was in danger of dying out. The Sanhedrin and some Jewish schools moved to Jamnia since Jerusalem was gone. It seems that this is the place where the Greek texts stopped being used and the Hebrew ones started being used exclusively. From Orthodox Apologetics:
"The word "deuterocanon" simply means "secondary". Or the secondary order of books. The historical title for the other set of books is "protocanon". Which means, "primary", or the Primary order of books. Some Orthodox Scholars prefer not to use the term "Deuterocanon" because that is a western Roman Catholic term. But whatever the case, in varying degrees, the Church has always embraced at least some of these books as scripture.
So when did the nonbelieving Jews officially reject the "Deuterocanon"?
Well, in 135 A.D. "Akiba ben Joseph" was made head of the Academy of Jamnia. It was under his influence that the Jews "officially" rejected the Deuto-canon."
So here the Christians are still using the Greek texts and the Deuterocanonicals - which are not rejected by the Jews until 100 years after Christ - and they continue to use the Septuagint for their translations even after the Masoretic Text came on the scene sometime around the 7th century AD. From Wikipedia:
"The MT was primarily copied, edited and distributed by a group of Jews known as the Masoretes between the 7th and 10th centuries CE. Though the consonants differ little from the text generally accepted in the early 2nd century (and also differ little from someQumran texts that are even older), it has numerous differences of both greater and lesser significance when compared to (extant 4th century) manuscripts of the Septuagint, a Greek translation (made in the 3rd to 2nd centuries BCE) of the Hebrew Scriptures that was in popular use in Egypt and Israel and that is believed by scholars to be the source often quoted in the New Testament."This is the text that most Protestant Bibles are based off of today. Notice that this translation did not come around until about seven hundred years - or later - after Christ. That means that during this time the Christian Church was using the Greek text (the Septuagint) and continued to use said text until the Protestant Reformation when a certain translator based his text off of the Hebrew text which he thought was older. Now I should note here that the Eastern Orthodox Church still uses the Septuagint as they did for 1500 years before the Reformation.
I find it amusing that most of the criticism I read about the "Apocrypha" is that the earliest manuscripts we have that contain all of them with the LXX is about the 5th or 6th century, and yet the earliest complete Jewish text we have with out the Apocrypha is from the 7th century (or later). Even copies of the Deuterocanonicals were found with the Dead Sea Scrolls which alludes to their use by the Jews of old.
Lastly I am going to touch on the date of the canon. Protestant apologists love to throw around the Council of Trent as to when the Roman Catholic Church "officially" adopted some of the Deuterocanonicals as canon. The dates for the Council of Trent are from 1545 to 1563 and of course the Reformation kicked off in 1517. The problem here is that the Catholic Church was not adding the Deuterocanonicals to their canon, they were reaffirming their place in the canon in response to the Protestants (who were supposed to have some part in the Council of Trent). Likewise, the Orthodox had the Synod of Jerusalem in 1672 which included the Deuterocanonicals, however this was probably a response to Protestants as the Deuterocanonicals have always been used by the Orthodox Church - though their use had been disputed by various Fathers with some claiming that they were not Scripture but were still good to read, and others claiming that they were Scripture - as can be attested to the canon stated in the Third Synod of Carthage in 397:
"The Council of Carthage, called the third by Denzinger, on 28 August 397 issued a canon of the Bible quoted as, "Genesis, Exodus, Leviticus, Numbers, Deuteronomy, Joshua son of Nun, Judges, Ruth, 4 books of Kingdoms, 2 books of Chronicles, Job, the Davidic Psalter, 5 books of Solomon, 12 books of Prophets, Isaiah, Jeremiah, Daniel, Ezekiel, Tobias, Judith, Esther, 2 books of Ezra, 2 books of Maccabees, and in the New Testament: 4 books of Gospels, 1 book of Acts of the Apostles, 13 letters of the Apostle Paul, 1 letter of his to the Hebrews, 2 of Peter, 3 of John, 1 of James, 1 of Jude, and one book of the Apocalypse of John.""So, as we can see there was a canon which included the Deuterocanonicals in AD 397. Of course, as I explained earlier, these books would have been part of the Septuagint as they had been since before the time of Jesus due to the majority of people speaking and understanding Greek.
Edit:: I have been asked to provide sources other than Wikipedia as Wikipedia is apparently not suitable to be used in academic writings. However, it is not my intention to sway anybody to the Traditions of the Orthodox Church, but to explain why we Orthodox have these Traditions. Nonetheless I have provided some other links that are not from Wikipedia* (The links I have provided are from a neutral standpoint, which is one reason I prefer Wikipedia as they try to ensure neutrality in their posts, whereas many other sites one can find will be biased one way or another).
Council of Carthage
Encyclopedia Judaica on Jabneh/Jamnia
On the Old Testament Canon - this link tries to be unbiased and has some other links that may be of interest on the right hand side.
*I believe their standards have become better, and if not for direct use in academic writings one could use the sources sited and external links found in which ever topic for academia.