The unifying themes of the Old Testament are the creation, covenant, holiness and redemption. God established his kingdom and glory through mankind. The creation of Eve and Adam was the first mission to establish his kingdom in earth. All the scriptures in the books of Old Testament have foundation in the creation to the fall then the Redemption. After the fall of humanity, God wanted to restore his nation through the promise to Abraham.
God was calling his people to be holly as he is Holy. He gave Moses commandments and Laws that Israelites should respect in order to differ from other nations. Then, the redemption was through Jesus Christ, the Son of God. Jesus is the Redeemer, and in the Old Testament he is called the Messiah. He is the one who connect all nations to God. 
I believe that the above mentioned themes are the core unifying of the Old Testament. They make the reason of all event of what happened in the Old Testament. All history of Israelites and the rest of humanity base on those themes.
 Waltke, Bruce. An Old Testament Theology: An Exegetical, Canonical, and Thematic Approach, Grand Rapids: Zondervan, 2006, 57
 Genesis 1: 26-28
 Longman, Tremper and Raymond B. Dillard, An Introduction to the Old Testament ( 2nd Edition), Grand Rapids: Zondervan, 2007
 Genesis 12-50
 2 Samuel 7:14 (ESV), Psalm 2:7 (ESV), Isaiah 7:14 (ESV), Hosea 11:1 (ESV), Micah 5:1–3 (ESV)