Perhaps you haven't heard me bang my 'Christ in the Old Testament' drum. (In which case you must be very new to the blog!). Welcome! Here's a half hour talk introducing the theme.
I consider the subject systematically, from the New Testament and from the Old Testament.
Systematically: Jesus really is the Word of the Father - if the OT saints knew God, they must have known Jesus.
From the NT: Jesus and the Apostles seemed to think He was known and trusted in OT times.
From the OT: Moses and the Prophets proclaim Christ in their own context on their own terms.
If you want to take the issue further, this page may help.