Onkelos is mentioned several times in the Talmud. According to the traditional Jewish sources, he was a prominent Roman nobleman, a nephew of the Roman emperor Titus. According to R' Eliyahu of Vilna he was a nephew of Hadrian, and not Titus. His conversion is the subject of a story whereupon he first consulted with the spirits of three deceased enemies of Israel to see how Israel fared in the next world (Gittin 56b). The first was his uncle Titus, who was blamed for the destruction of the Second Temple; the second was the seer Balaam, hired by Balak king of Moab to curse Israel; and the last was Yeshu, a name used for those who sought to lead Jews astray to idolatry, in particular an idolatrous former student of Rabbi Joshua ben Perachiah in the Hasmonean period as well as king Manasseh of Judah. (In later writings Yeshu is used for Jesus, but opinions differ over whether it can be understood this way in the Talmud.) Onkelos is said to have seen all of them subjected to humiliating punishments for harming Israel. The earlier Jerusalem Talmud gives the subject of these stories as Aquila, another convert to Judaism, who translated the Bible into Greek. Stories about the two men had become confounded due to the similarity of names. By another theory, "Onkelos" is simply a variant of "Aquila", applied in error to the Aramaic instead of the Greek translation.
After his conversion, the Talmud records a story of how the Roman emperor tried to have Onkelos arrested (Avodah Zarah 11a). Onkelos cited verses from the Tanakh to the first Roman legion, who then converted. The second legion was also converted, after he juxtaposed God's personal guidance of Israel in the Book of Numbers to the Roman social hierarchy. A similar tactic was used for the third legion, where Onkelos compared his mezuzah to a symbol of God guarding the home of every Jew, in contrast to a Roman king who has his servants guard him. The third legion also converted and no more were sent.
According to tradition, Onkelos authored his Targum, i.e.Targum Onkelos as an exposition of the "official" interpretation of the pshat (or basic meaning) of the Torah, as received by Rabbi Eliezer. This helped canonise the status of both Onkelos and his Targum in the Jewish tradition.