The Septuagint is the Old Testament as used by the Apostles and all the Ancient Church
Elpenor's Bilingual (Greek / English) Old Testament
English translation by L.C.L. Brenton
Page 2: Introduction to the Septuagint Old Testament
HE Greek version of the Old Testament commonly known as the Septuagint (...) is a version of a Hebrew text earlier by about a millennium than the earliest dated Hebrew manuscript extant (916 AD), a version, in particular, prior to the formal rabbinical revision of the Hebrew which took place early in the 2nd century AD. (...) It is, moreover, a pioneering work; there was probably no precedent in the world's history for a series of translations from one language into another on so extensive a scale. It was the first attempt to reproduce the Hebrew Scriptures in another tongue. It is one of the outstanding results of the breaking-down of international barriers by the conquests of Alexander the Great and the dissemination of the Greek language, which were fraught with such vital consequences for the history of religion. The cosmopolitan city which he founded in the Delta witnessed the first attempt to bridge the gulf between Jewish and Greek thought. The Jewish commercial settlers at Alexandria, forced by circumstances to abandon their language, clung tenaciously to their faith; and the translation of the Scriptures into their adopted language, produced to meet their own needs, had the further result of introducing the outside world to a knowledge of their history and religion. Then came the most momentous event in its history, the starting-point of a new life; the translation was taken over from the Jews by the Christian church.
Septuagint Random Chapter
Read about the texts witnessed only in the Septuagint:
Letter of Aristeas (including also full text in Greek and English) ||| Sirach, Wisdom, Letter of Jeremiah ||| Maccabees 1, 2, 3 and 4 ||| Additional texts witnessed by the Septuagint to the book of Daniel ||| 1 Esdras, Psalm 151, Prayer of Manasseh ||| Judith, Baruch
Note that the so called 'sixth' chapter of Baruch in the Septuagint is published separately as Letter of Jeremiah. Check also this note about the Order of Septuagint Psalms and the Masoretic.
Cf. in print A New English translation of the Septuagint, Greek English Lexicon of the Septuagint, Grammar of Septuagint Greek, The Use of the Septuagint in New Testament Research, More
Reference address : https://ellopos.net/elpenor/greek-texts/septuagint/default.asp?pg=2