Who Was The Old Testament God?
This is actually a simple question. There should not be any confusion over it.
Let's notice one scripture, a quote from Jesus:
Note first that no one knew the Son, which is to say that no one recognized Jesus as being God. Secondly, that no one knew the Father (except the Son, and those he chose to reveal him to). So the Jews were not aware of the Father, save those to whom Jesus chose to reveal him.
Now let's move on to the first chapter of John.
So the world, and the universe was made by the Word. This was the God-entity that created everything.
Moving on to verse 14:
So the same God-being that created everything is the same as the one who became flesh, which is to say Jesus. In other words, Jesus was the God of the Old Testament. God the Father was unknown to the Old Testament peoples, for no one knew of his existence. It was hinted at, but most people would not have been aware of it.
The Old Testament has several names or titles for God. The first reference is in the very first verse of the Bible.
The word God in the above verse is translated from the Hebrew word Elohim. The 'im' ending in Hebrew implies plurality, more than one.
There is an interesting Psalm, written by King David.
The God of King David is my Lord. So who is the LORD?
Now, let's look at a passage from the Old Testament, compared to one from the New Testament.
God was identifying himself to Moses as I AM, or the self-existent one. Now let's notice a conversation by Jesus with the Jews.
Why were the Jews going to stone Jesus? It's because they understood what he said, "Before Abraham was, I am" (or I self-existed). Jesus was telling them in no uncertain terms that he was the one who spoke to Moses. He was the God of their fathers. To the Jews, this would have been considered blasphemy, and they would not have waivered in picking up stones.
Paul confirms that Jesus was the God of the Old Testament in his epistles.