The O antiphon for Dec. 20th begins by addressing our Savior as O Clavis David, O Key of David. The antiphon then describes what the Key of David symbolizes, and it ends with a petition, "come and lead the prisoners from the prison house, that sit in darkness and the shadow of death.”
O Clavis David, et sceptrum domus Israel; qui aperis, et nemo claudit; claudis, et nemo aperit: veni, et educ vinctum de domo carceris, sedentem in tenebris, et umbra mortis.
O Key of David and sceptre of the House of Israel; who openest and no man shutteth; shuttest and no man openeth: come and lead the prisoners from the prison house, that sit in darkness and the shadow of death.
Here are the corresponding modified hymn verses from "Veni, Veni, Emmanuel" and "O Come, O Come Emmanuel."
Veni clavis Davidica! Regna reclude coelica, Fac iter Tutum superum, Et claude vias Inferum. R: Gaude! Gaude! Emmanuel, nascetur pro te Israel.
O come, Thou Key of David, come And open wide our heav'nly home; Make safe the way that leads on high, And close the path to misery. R: Rejoice ! Rejoice ! Emmanuel Shall come to thee, O Israel.
This title of the Messiah is based on many Scripture texts.
Isaiah 9:6-7: "His dominion is vast and forever peaceful, Upon David’s throne, and over his kingdom, which he confirms and sustains By judgment and justice, both now and forever. The zeal of the LORD of hosts will do this!"
Isaiah 22:22: "I will place the key of the House of David on his shoulder. When he opens, no one shall shut; when he shuts, no one shall open."
Christ is from the line of David and is the heir of David as King of Israel, the heavenly kingdom, in an unforeseen way. The ruler wore the keys, the symbol of his power and authority, on his shoulder.
Revelation 3:7: "To the presiding spirit of the church in Philadelphia write this: 'The holy One, the true, who wields David's key, who opens and no one can close, who closes and no one can open.'"
Christ, the holy one and true, is the heir of David. All power and authority was given to him after the resurrection. He gave the power to "bind and to loose" our sins to the Apostles.
Matthew 16:18-19: "And so I say to you, you are Peter, and upon this rock I will build my church, and the gates of the netherworld shall not prevail against it. I will give you the keys to the kingdom of heaven. Whatever you bind on earth shall be bound in heaven; and whatever you loose on earth shall be loosed in heaven.”
Jesus is the One who unlocks the fetters of sin that keep us tightly chained and who frees us from our captivity.
Psalm 107: 10, 14: "they dwelt in darkness and gloom, bondsmen in want and in chains,...and he led them forth from darkness and gloom and broke their bonds asunder."
Here is Dom Gueranger’s commentary about this O antiphon for December 20.
"O Jesus, Son of David! heir to his throne and his power! Thou art now passing over, in Thy way to Bethlehem, the land that once was the kingdom of Thy ancestor, but now is tributary to the Gentiles. Scarce an inch of this ground which has not witnessed the miracles of the justice and mercy of Jehovah, Thy Father, to the people of the old Covenant, which is so soon to end. Before long, when Thou hast come from beneath the virginal cloud which now hides Thee, Thou wilt pass along this same road doing good, 1 healing all manner of sickness and every infirmity, and yet having not where to lay Thy head. Now, at least, Thy Mother’s womb affords Thee the sweetest rest, and Thou receivest from her the profoundest adoration and the tenderest love. But, dear Jesus, it is Thine own blessed will that Thou leave this loved abode. Thou hast, O eternal Light, to shine in the midst of this world’s darkness, this prison where the captive, whom Thou hast come to deliver, sits in the shadow of death. Open his prison-gates by Thy all-powerful key. And who is this captive, but the human race, the slave of error and vice? Who is this captive, but the heart of man, which is thrall to the very passions it blushes to obey? Oh! come and set at liberty the world Thou hast enriched by Thy grace, and the creatures whom Thou hast made to be Thine own brethren.'
Here is a link from New Liturgical Movement that includes a link to a sound file of the chant.
History and Mystery: The O Antiphons in a Favorite Hymn" for my post on the relationship between the O antiphons,"Veni, Veni, Emmanuel" and "O Come, O Come Emmanuel."