From The Book of Common Worship’s Times and Seasons (p.58).

These antiphons, or refrains, all beginning ‘O …’, were sung before and after the Magnificat at Vespers, according to the Roman use, on the seven days preceding Christmas Eve (17–23 December). They are addressed to God, calling for him to come as teacher and deliverer, with a tapestry of scriptural titles and pictures that describe his saving work in Christ. In the medieval rite of Salisbury Cathedral that was widely followed in England before the Reformation, the antiphons began on 16 December and there was an additional antiphon (‘O Virgin of virgins’) on 23 December; this is reflected in the Calendar of The Book of Common Prayer, where 16 December is designated O Sapientia (O Wisdom). The Common Worship Calendar has adopted the more widely used form. It is not known when and by whom the antiphons were composed, but they were already in use by the eighth century.

23 December – O Emmanuel

O Emmanuel, our King and our lawgiver, the hope of the nations and their Savior: Come and save us, O Lord our God.

—cf Isaiah 7.14

From The Book of Common Worship’s Times and Seasons (p.58).

These antiphons, or refrains, all beginning ‘O …’, were sung before and after the Magnificat at Vespers, according to the Roman use, on the seven days preceding Christmas Eve (17–23 December). They are addressed to God, calling for him to come as teacher and deliverer, with a tapestry of scriptural titles and pictures that describe his saving work in Christ. In the medieval rite of Salisbury Cathedral that was widely followed in England before the Reformation, the antiphons began on 16 December and there was an additional antiphon (‘O Virgin of virgins’) on 23 December; this is reflected in the Calendar of The Book of Common Prayer, where 16 December is designated O Sapientia (O Wisdom). The Common Worship Calendar has adopted the more widely used form. It is not known when and by whom the antiphons were composed, but they were already in use by the eighth century.

22 December – O Rex Gentium

O King of the nations, and their desire, the cornerstone making both one: Come and save the human race, which you fashioned from clay.

—cf Isaiah 28.16; Ephesians 2.14

From The Book of Common Worship’s Times and Seasons (p.58).

These antiphons, or refrains, all beginning ‘O …’, were sung before and after the Magnificat at Vespers, according to the Roman use, on the seven days preceding Christmas Eve (17–23 December). They are addressed to God, calling for him to come as teacher and deliverer, with a tapestry of scriptural titles and pictures that describe his saving work in Christ. In the medieval rite of Salisbury Cathedral that was widely followed in England before the Reformation, the antiphons began on 16 December and there was an additional antiphon (‘O Virgin of virgins’) on 23 December; this is reflected in the Calendar of The Book of Common Prayer, where 16 December is designated O Sapientia (O Wisdom). The Common Worship Calendar has adopted the more widely used form. It is not known when and by whom the antiphons were composed, but they were already in use by the eighth century.

21 December – O Oriens

O Morning Star, splendor of light eternal and sun of righteousness: Come and enlighten those who dwell in darkness and the shadow of death.

—cf Malachi 4.2

From The Book of Common Worship’s Times and Seasons (p.58).

These antiphons, or refrains, all beginning ‘O …’, were sung before and after the Magnificat at Vespers, according to the Roman use, on the seven days preceding Christmas Eve (17–23 December). They are addressed to God, calling for him to come as teacher and deliverer, with a tapestry of scriptural titles and pictures that describe his saving work in Christ. In the medieval rite of Salisbury Cathedral that was widely followed in England before the Reformation, the antiphons began on 16 December and there was an additional antiphon (‘O Virgin of virgins’) on 23 December; this is reflected in the Calendar of The Book of Common Prayer, where 16 December is designated O Sapientia (O Wisdom). The Common Worship Calendar has adopted the more widely used form. It is not known when and by whom the antiphons were composed, but they were already in use by the eighth century. 

20 December – O Clavis David

O Key of David and sceptre of the House of Israel; you open and no one can shut; you shut and no one can open: Come and lead the prisoners from the prison house, those who dwell in darkness and the shadow of death.

—cf Isaiah 22.22; 42.7

From The Book of Common Worship’s Times and Seasons (p.58).

These antiphons, or refrains, all beginning ‘O …’, were sung before and after the Magnificat at Vespers, according to the Roman use, on the seven days preceding Christmas Eve (17–23 December). They are addressed to God, calling for him to come as teacher and deliverer, with a tapestry of scriptural titles and pictures that describe his saving work in Christ. In the medieval rite of Salisbury Cathedral that was widely followed in England before the Reformation, the antiphons began on 16 December and there was an additional antiphon (‘O Virgin of virgins’) on 23 December; this is reflected in the Calendar of The Book of Common Prayer, where 16 December is designated O Sapientia (O Wisdom). The Common Worship Calendar has adopted the more widely used form. It is not known when and by whom the antiphons were composed, but they were already in use by the eighth century.

19 December – O  Radix Jesse

O Root of Jesse, standing as a sign among the peoples; before you kings will shut their mouths, to you the nations will make their prayer: Come and deliver us, and delay no longer.

—cf Isaiah 11.10; 45.14; 52.15; Romans 15.12 

From The Book of Common Worship’s Times and Seasons (p.58).

These antiphons, or refrains, all beginning ‘O …’, were sung before and after the Magnificat at Vespers, according to the Roman use, on the seven days preceding Christmas Eve (17–23 December). They are addressed to God, calling for him to come as teacher and deliverer, with a tapestry of scriptural titles and pictures that describe his saving work in Christ. In the medieval rite of Salisbury Cathedral that was widely followed in England before the Reformation, the antiphons began on 16 December and there was an additional antiphon (‘O Virgin of virgins’) on 23 December; this is reflected in the Calendar of The Book of Common Prayer, where 16 December is designated O Sapientia (O Wisdom). The Common Worship Calendar has adopted the more widely used form. It is not known when and by whom the antiphons were composed, but they were already in use by the eighth century.

December 18 — O Adonai

O Adonai, and leader of the House of Israel,
who appeared to Moses in the ?re of the burning bush
and gave him the law on Sinai:
Come and redeem us with an outstretched arm.

—cf Exodus 3.2; 24.12

An antiphon is (in traditional Western Christian liturgy) a short sentence sung or recited before or after a psalm or canticle. Today begins the Advent Antiphons. But what are the “O Antiphons”? Below is an excerpt from the Catholic Education Resource Center by Father William Saunders. I wholeheartedly commend their use each of these seven days.

The “O Antiphons” refer to the seven antiphons that are recited (or chanted) preceding the Magnificat [Song of Mary] during Vespers [Evening Prayer] of the [Roman Catholic] Liturgy of the Hours. They cover the special period of Advent preparation known as the Octave before Christmas, Dec. 17-23, with Dec. 24 being Christmas Eve and Vespers for that evening being for the Christmas Vigil.

The exact origin of the “O Antiphons” is not known. Boethius (c. 480-524) made a slight reference to them, thereby suggesting their presence at that time. At the Benedictine abbey of Fleury (now Saint-Benoit-sur-Loire), these antiphons were recited by the abbot and other abbey leaders in descending rank, and then a gift was given to each member of the community. By the eighth century, they are in use in the liturgical celebrations in Rome. The usage of the “O Antiphons” was so prevalent in monasteries that the phrases, “Keep your O” and “The Great O Antiphons” were common parlance. One may thereby conclude that in some fashion the “O Antiphons” have been part of our liturgical tradition since the very early Church.

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.

Read the whole article.

December 17 — O Sapientia (O Wisdom)
O Wisdom, coming forth from the mouth of the Most High,
reaching from one end to the other mightily,
and sweetly ordering all things:
Come and teach us the way of prudence.

—cf Ecclesiasticus 24.3; Wisdom 8.1