I am not in the inner circle of people who have academic credentials to qualify them to discuss this subject.
Lacking that, I have read Catholic books that say the Septuagint is the official Old Testament of the Roman Catholic Church.
In reality, I think it’s a half-truth. Yes, the Greek translations were
the Old Testament for most Christians during the first three centuries of Christianity; in fact, it (and not the Hebrew text of the OT) was the base of Christian theology and piety.
Only a few people were really aware that the OT they’re reading was different from the OT text the Jews were reading, and fewer cared (though most of them could not read Hebrew). Those who cared quickly found out that the OT text they’re using was not exactly the same as the Jews had, and many of them explained these differences by either blaming the Jews (‘They altered the Scriptures to remove prophecies about Jesus from it!’

) or believing that these differences were stumbling blocks placed by God in order to drive the reader into a deeper meaning.
Most, however, were not concerned about whether the Greek was a faithful translation of the Hebrew or not (not that they could tell anyway); as far as they are concerned, the Greek OT is ‘our’ Bible. The Jews might be using a somewhat different OT, but for us, the Greek OT is ‘our’ Scriptures. In fact, they accorded a sort of inspired status for the Greek OT too; in their view, God may have given his original revelation in Hebrew and Aramaic, but in order to prepare the world for Jesus’ coming (and foreseeing that the Jews would reject Him) they thought that God ‘spoke Greek’.
However, starting from the 3rd century onwards there was a sort of ‘revolutionary’ change within the Church: Origen published the so-called
Hexapla, a word-by-word comparison of four Greek versions of the OT plus the (proto-Masoretic) Hebrew text and a transliteration of it into Greek letters in six columns (hence the name). One of these four translations was the Old Greek (aka ‘Septuagint’) text with interpolations to indicate where the Hebrew is not represented in the Greek and marks indicating places in the text where the Greek has additional words not found in Hebrew.
Origen gives two reasons for this project: in one place, he claims the project had apologetic motives: when discussing texts he could compare different versions to be sure that he was not building his argument on a text Jews did not accept. In another place, he claims the project was done in order to ‘heal’ the Greek text; he thought that the proto-Masoretic text - which had already become the standard text among Jews of his day - is the source text of the Greek OT. (We now know that different Hebrew texts underlie many of the ‘Septuagint’ OT books.)
Origen never set out to disparage the ‘Septuagint’ or even to dislodge its position from the Church, but the
Hexapla really kickstarted the death of the ‘Septuagint’ in (Western) Christianity. His attention to the Hebrew text later led some scholars to wonder whether or not the Church had been missing out by ignoring it. His hybrid ‘fifth column’ - the edited ‘Septuagint’ - was copied frequently, eventually without the editing marks. Origen’s text ‘contaminated’ the stream of transmission: from the 4th century onwards almost all manuscripts of the ‘Septuagint’ was influenced by the so-called Hexaplaric or Origenic version.
Origen began the process (even if somewhat accidentally); it would be the three Eusebiuses from the 4th-5th century who would deliver the finishing blows. First, you have Eusebius of Caesarea and his teacher Pamphilus, both instrumental in disseminating the Hexaplaric recension of the ‘Septuagint’, now without the editing marks.
Second is Eusebius of Emesa, who studied under the Caesarean Eusebius; unlike Origen or the other Eusebius, this Eusebius clearly had a preference for the Hebrew text and sought to elevate it, only stopping short of a full endorsement of it over the ‘Septuagint’. It was due to his mixed heritage, really; he knew Greek
and Syriac, and thus would had a heightened awareness that the Greek OT was but a mere translation, and one that was different from the Hebrew text Jews were using. (Syriac Christians, uniquely among Christians back then, used
an OT text that was translated mainly from the Hebrew.)
The third and final Eusebius would be Eusebius Sophronius Hieronymus - aka Jerome. Pretty much the ultimate Hebrew lover and supporter of
hebraica veritas (‘the truth of the Hebrew (text)’) of early Christianity. (It seems he was also influenced by Eusebius of Emesa.) St. Jerome’s translations of OT books
from the Hebrew - a very controversial move - was the final stroke against the Septuagint in the West (represented by its translations into Latin).
Ever since Jerome we Western Christians have really had a mixed heritage: the mark of the ‘Septuagint’ still remains in our theology and piety, but the actual Scriptures we’ve been reading is essentially a hodge-podge of the Hebrew and the Greek.