The Old Testament is not monotheistic (neither is the New Testament, honestly) is by monotheism you mean belief in only one, completely unified divinity worthy of worship alone. Why do you think Jewish binitarianism exists in the Hekhalot literature? How do you think we got the Trinity?
It's freakin jewish.
The biggest examples of this are the Metatron traditions in Jewish mysticism: a quasi-divine agent of God, sometimes called the lesser YHWH.