Targum Onkelos is the official eastern (Babylonian) targum to the Torah. Its early origins, however, are western (Land of Israel). It is considered by Orthodox Jews to have been written by Onkelos, a statement disputed by many secular/liberal scholars.
In Talmudic times (and to this day in Yemenite Jewish communities) Targum Onkelos was read as a verse-by-verse translation alternatively with the Hebrew verses of the Torah in the synagogue.
The Talmud states that "a persion should complete his portions of scripture along with the community, reading the scripture twice and the targum once." This passage is taken to refer to Targum Onkelos.