(excerpts from The Liturgical Year, vol 1 by Dom Guerenger)
On December 17, the Church enters upon on the seven days which precede the Vigil of Christmas, and which are known in the liturgy under the name of the Greater Ferias. The ordinary of the Advent Office becomes more solemn; the antiphons of the psalms, both for Lauds and the Hours of the day, are proper, and allude expressly to the great coming. Every day, at Vespers, is sung a solemn antiphon, consisting of a fervent prayer to the Messias, whom it addresses by one of the titles given Him in the Sacred Scriptures.
In the Roman Church, there are seven of these antiphons, one for each of the greater ferias. They are commonly called the O’s of Advent, because they all begin with that interjection…
The canonical Hour of Vespers has been selected as the most appropriate time for this solemn supplication to our Saviour, because, as the Church sings in one of her hymns, it was in the evening of the world (vergente mundi vespere) that the Messias came amongst us. These antiphons are sung at the Magnificat, to show us that the Saviour whom we expect is to come to us by Mary. They are sung twice, once before and once after the canticle, as on double feasts, and this to show their great solemnity. In some Churches it was formerly the practice to sing them thrice; that is, before the canticle, before the Gloria Patri, and after the Sicut erat. Lastly, these admirable antiphons, which contain the whole pith of the Advent liturgy, are accompanied by a chant replete with melodious gravity, and by ceremonies of great expressiveness, though in these latter, there is no uniform practice followed. Let us enter into the spirit of the Church; let us reflect on the great day which is coming; that thus we may take our share in these the last and most earnest solicitations of the Church imploring her Spouse to come, to which He at length yields.
I. — O Wisdom, that proceedest from the mouth of the Most High, reaching from end to end mightily, and disposing all things sweetly! Come and teach us the way of prudence.
II. — O Adonaï, and leader of the house of Israel, who appearedst to Moses in the fire of the flaming bush, and gavest him the law on Sinaï; come and redeem us by thy outstretched arm.
III. — O Root of Jesse, who standest as the ensign of the people; before whom kings shall not open their lips; to whome the nations shall pray: come and deliver us; tarry now no more.
IV. — O Key of David, and sceptre of the house of Israel! Who openest, and no man shutteth: who shuttest, and no man openeth; come, and lead the captive from prison, sitting in darkness and in the shadow of death.
V. — O Orient! Splendor of the eternal light, and Sun of justice! Comw and enlighten them that sit in darkness, and in the shadow of death.
VI. — O King of Nations, and their desired One, and the cornerstone that makest both one; come and save man whom thou formedst out of slime.
VII. — O Emmanuel, our King and Lawgiver, the Expectation and Saviour of the nations! Come and save us, O Lord our God!