Supposing you were planning an eight part series from the Old Testament, what would you cover? The point would be to see the gospel unfolded, and as Martin Downes shows - it is the Old Testament that gives us the meaning of the cross and resurrection which happened 'according to the scriptures'.
Realistically you've got to hit Genesis 12 (promise to Abraham of a global blessing through Jesus), Leviticus 16 (the day of atonement which Jesus fulfills), 2 Samuel 7 (the promise of great David's greater son) and Isaiah 53 (the suffering servant). But what else goes in?
Here's an idea...
(1) Genesis 1 The Word Shines (how the word of God forms and fills the creation and anticipates the new creation in which we see Jesus)
(2) Genesis 2-3 The Tree of Life (and how a serpent-crushing seed will get us past the angels to it)
(3) Genesis 17:7 The Heir: Inheritor of all things (that all things point to Jesus, all promises are his)
(4) Exodus 26 The Pattern: Exactly as you see it (on the tabernacle, and meaning thereof)
(5) Leviticus 16 The Day of Atonement (sin, guilt and wrath dealt with)
(6) Leviticus 25 The Day of Jubilee (he comes out to blow the trumpet)
(7) 2 Samuel 7 The Great King (the promised seed is God's son who rules forver!)
(8) Isaiah 53 The Suffering Servant (he dies for us)
But that leaves out some amazing stuff.... maybe it's best just to go book by book, The Gospel According to Genesis etc? Maybe we could just teach Matthew or Mark and unpack the meaning from the Old Testament? Really coming to Christ is the lifetime exercise of studying all Scripture so such a series is a bit unnecessary but sometimes it helps to flesh out the storyline from a high altitude. How would you do it? What would your series be?