In Exodus 3, the angel of the LORD appeared to Moses in the burning bush, and God says to Moses, "I AM WHO I AM." He describes himself again as the "God of your fathers, the God of Abraham, the God of Isaac, and the God of Jacob." It seems quite clear that in the Old Testament, God is "the God of your fathers."
But something changed with the dawn of the kingdom. In Matthew 6:9-13, Jesus teaches his followers to address God himself as Father. This is clearly a shift from the relationship the Israelite people shared with "I AM."
"Father, thank you for adopting me to be your son, and paying for my adoption with the sacrifice of my elder brother, my LORD, my God!"