A big chunk of Genesis is lifted from Gilgamesh, especially the major themes. Paradise lost and a great flood, etc.
Ecclesiastes is almost a direct lift of parallel themes from Gilgamesh. In both cases it is a narration about the meaningless of life in the face of death.
Then of course the OT itself is a composite of several religions carried to Moses via oral tradition, including an Egyptian religion which laid the foundation of a belief in the Oneness of God, resurrection, etc.
Jewish tradition has a system called Midrash which includes transposing timelines, so that past events come into present events and much of Jesus back story got lifted from the OT to complete His story.
