Lilly,
There are a number of New Testament passages that make it clear that it is the "Father" that is the God within the Old Testament. Already Hebrews 1:1-2 has been discussed. We see that "the God" that "spoke by the prophets" was the Father, not the Son. For in Hebrews 1 we read of "the God" that did this and in verse 2 we read of the son of that God.
Let us consider Hebrews 3:4-5: "For every house is built by someone; but He who built all things is God. And "Moses" truly "was faithful in all his house" as a ministering servant, for a testimony of the things having been spoken; "
God built everything and Moses was "faithful in all his house." This text is quote from Numbers 12:7, where God states: "It is not so with My servant Moses! He is faithful in all My house." Going back to Hebrews 3, we can continue into verse 6: "but Christ as Son over His house, whose house we are, if truly we hold fast the boldness and rejoicing of the hope firm to the end." Christ was not the God who possessed the house, who was speaking in Num. 12:7, but the Son of that God.
Also, Acts 3:15, which quotes from Exodus 3:15, refering to the God that spoke in that text. We are there told that Jesus is "his servant," which is the servant of the God that spoke in Exodus 3:15, not that God himself or a person of that God.
Further, Acts 3:25-26, which quotes God speaking to Abraham in Genesis 22:18. We see that Jesus isn't that God or a person of that God, because we read of that God who spoke there, and again Jesus is said to be that one's servant. Acts 5:30-31 supports this too, also referencing that text.
Finally, we might consider Jesus' words of John 8:54. "Jesus answered, If I glorify Myself, My glory is nothing; it is My Father who glorifies Me, whom you say is your God."
The God of the Jews, the God of the Old Testament, according to Jesus, was not a Trinity or Jesus himself, but that one was his Father. He is the God of the Old Testament, the Jehovah who alone is the most high. (Psa. 83:18)
Mondo