As the use of Aramaic declined, the practice of reciting the Targum in the synagogue fell into disuse in most Jewish communities.
The name "Onkelos" was attached to the present work in early medieval times on account of a mistaken identification with a translation by "Onkelos the Proselyte" that is mentioned in the Talmud. It is clear that the Talmudic reference is really to the Greek translation of the Torah by Aquila, portions of which are cited in the Palestinian Talmud and in Christian sources.
The current Aramaic translation has no known author, and was evidently the standard version that was in use in Babylonian synagogues during the Talmudic era. Several quotations of the Targum in the Babylonian Talmud agree with our "Targum Onkelos"; most of them are brought in the name of the third-century Babylonian scholar Rav Joseph, indicating perhaps that he took an active part in its compilation.
The Aramaic dialect of Targum Onkelos seems to be that of second-century Israel, though many scholars believe that it underwent subsequent development in Babylonia during the Talmudic era.
Israel and Babylonia
For Biblical sections such as the testaments of Jacob (end of Genesis) and Moses (end of Deuteronomy), Targum Onkelos renders these with expansive homiletical interpretations, analogous to the style of the Palestinian Targums.
Targum Onkelos was uncomfortable with Hebrew expressions that suggest direct interaction between God and his creatures. In some cases it gets around these difficulties through circumlocutions. Thus instead of speaking to God, Moses usually speaks before God.
Similarly, Onkelos introduces the "word" (memra) of God as an intermediary between God and the world, an approach which seems to echo the use of the "logos" in the writings of the Alexandrian Jewish philosopher Philo.