From as early as the fifth century, Christians have recited, chanted or prayed the "O" Antiphons as part of their evening vespers (prayers) in the octave before Christmas from December 17 to the 23. Each antiphon is from Isaiah's prophecy of the coming Messiah. They are preceded with the exclamation "Oh", hence the name, the "O" antiphons. The beautiful advent hymn, O Come, O Come Emmanuel annunciates each antiphon.
The following explanation is from Father William Saunders:
The importance of “O Antiphons” is twofold: Each one highlights a title for the Messiah: O Sapientia (O Wisdom), O Adonai (O Lord), O Radix Jesse (O Root of Jesse), O Clavis David (O Key of David), O Oriens (O Rising Sun), O Rex Gentium (O King of the Nations), and O Emmanuel. Also, each one refers to the prophecy of Isaiah of the coming of the Messiah. Let’s now look at each antiphon with just a sample of Isaiah’s related prophecies :
O Sapientia: “O Wisdom, O holy Word of God, you govern all creation with your strong yet tender care. Come and show your people the way to salvation.” Isaiah had prophesied, “The spirit of the Lord shall rest upon him: a spirit of wisdom and of understanding, a spirit of counsel and of strength, a spirit of knowledge and fear of the Lord, and his delight shall be the fear of the Lord.” (11:2-3), and “Wonderful is His counsel and great is His wisdom.” (28:29).
O Adonai: “O sacred Lord of ancient Israel, who showed yourself to Moses in the burning bush, who gave him the holy law on Sinai mountain: come, stretch out your mighty hand to set us free.” Isaiah had prophesied, “But He shall judge the poor with justice, and decide aright for the land’s afflicted. He shall strike the ruthless with the rod of his mouth, and with the breath of his lips he shall slay the wicked. Justice shall be the band around his waist, and faithfulness a belt upon his hips.” (11:4-5); and “Indeed the Lord will be there with us, majestic; yes the Lord our judge, the Lord our lawgiver, the Lord our king, he it is who will save us.” (33:22).
O Radix Jesse: “O Flower of Jesse’s stem, you have been raised up as a sign for all peoples; kings stand silent in your presence; the nations bow down in worship before you. Come, let nothing keep you from coming to our aid.” Isaiah had prophesied, “But a shoot shall sprout from the stump of Jesse, and from his roots a bud shall blossom.” (11:1), and A On that day, the root of Jesse, set up as a signal for the nations, the Gentiles shall seek out, for his dwelling shall be glorious.” (11:10). Remember also that Jesse was the father of King David, and Micah had prophesied that the Messiah would be of the house and lineage of David and be born in David’s city, Bethlehem (Micah 5:1).
O Clavis David: “O Key of David, O royal Power of Israel controlling at your will the gate of Heaven: Come, break down the prison walls of death for those who dwell in darkness and the shadow of death; and lead your captive people into freedom.” Isaiah had prophesied, AI will place the Key of the House of David on His shoulder; when he opens, no one will shut, when he shuts, no one will open.” (22:22), and “His dominion is vast and forever peaceful, from David’s throne, and over His kingdom, which he confirms and sustains by judgment and justice, both now and forever.” (9:6).
O Oriens: “O Radiant Dawn, splendor of eternal light, sun of justice: come, shine on those who dwell in darkness and the shadow of death.” Isaiah had prophesied, “The people who walked in darkness have seen a great light; upon those who dwelt in the land of gloom a light has shown.” (9:1).
O Rex Gentium: “O King of all the nations, the only joy of every human heart; O Keystone of the mighty arch of man, come and save the creature you fashioned from the dust.” Isaiah had prophesied, “For a child is born to us, a son is given us; upon his shoulder dominion rests. They name him Wonder-Counselor, God-Hero, Father-Forever, Prince of Peace.” (9:5), and “He shall judge between the nations, and impose terms on many peoples. They shall beat their swords into plowshares and their spears into pruning hooks; one nation shall not raise the sword against another, nor shall they train for war again.” (2:4) .
O Emmanuel: “O Emmanuel, king and lawgiver, desire of the nations, Savior of all people, come and set us free, Lord our God.” Isaiah had prophesied, “The Lord himself will give you this sign: the Virgin shall be with child, and bear a son, and shall name him Emmanuel.”
(7:14). Remember “Emmanuel” means “God is with us.”
According to Professor Robert Greenberg of the San Francisco Conservatory of Music, the Benedictine monks arranged these antiphons with a definite purpose. If one starts with the last title and takes the first letter of each one - Emmanuel, Rex, Oriens, Clavis, Radix, Adonai, Sapientia - the Latin words ero cras are formed, meaning, “Tomorrow, I will come.” Therefore, the Lord Jesus, whose coming we have prepared for in Advent and whom we have addressed in these seven Messianic titles, now speaks to us, “Tomorrow, I will come.” So the “O Antiphons” not only bring intensity to our Advent preparation, but bring it to a joyful conclusion.
What wonderful tools our Church gives us to increase our devotion to Christ at this holy season!