O Root of Jesse’s stem,
sign of God’s love for all his people:
come to save us without delay!
In the second book of Samuel, King David expresses to the prophet Nathan his desire to build a house for the Ark of the Covenant. In other words, David wants to build a house for God. But as is so often true of human endeavors in the presence of God, it is God who turns this desire of David on its head. It is not David who will build a house for God, but God who will build a house for David. While David is a king in an earthly sense, it is important to remember that God is the ultimate king for his people.
To make sure we understand it is Jesus who fulfills the promise made by the Father, we are reminded that Jesus, Emmanuel, like David, comes from the root of Jesse. As king, David holds a privileged place in Jewish history, and so reference to him would have been powerful. But as great as David was, David was also a flawed figure in human history. Great was his dedication, but great too were his failures. Therefore, God makes the promise about his son. Unlike King David, the Son of God will be great and unflawed. Unlike David, whose people paid the price for David’s sin, Jesus will take on the sin of the people for their salvation. Jesus becomes the sign of God’s salvation, a saving action of grace that is far more powerful than any other.
The USCCB has a short description of the “O Antiphons” here.