O Antiphons

The opening words of the seven antiphons, of the Roman Breviary, to the Magnificat, for vespers, of the seven days preceding the vigil of Christmas are:

O Sapientia (O Wisdom)
O Adonai (O Adonai)
O Radix Jesse (O Root of Jesse)
O Clavis David (O Key of David)
O Oriens (O Morning Star)
O Rex Gentium (O King of the nations)
O Emmanuel (O Emmanuel)

Boëthius, fl.c. 500 a.D., makes a reference to the O antiphons. The Benedictines, are required to, sing or recite the breviary daily. These seven antiphons are sung at the end of the Advent season. From time to time and place to place, the antiphons were added to or substituted, but classically, the above are the seven.
The last antiphon complete is:

O Emmanuel, Rex et legifer noster, exspectatio Gentium, et Salvator earum:
veni ad salvandum nos, Domine, Deus noster.

O Emmanuel, our King and our Lawgiver, the Expected of the nations, and their Savior:
come to save us, O Lord, our God.

Now, the antiphons all refer back to Scripture. The last one to the 1st half of Jeremias x. 7:

Quis non timebit te, o Rex gentium ? tuum est enim decus :
Who shall fear thee, O king of nations? for thine is the glory:

There are several references, for each, to both Testaments, more from Isaias than any other book. Isaias is the prophet of Advent. Each are addressed to the Messias, by an equivalent title as a name. These antiphons congeal easily to an hymn, O Come, O Come, Immanuel (Veni, veni, Emmanuel). In the mid 19th century, John Mason Neale made an english translation.

These antiphons may have entered ☧ian usage through jewish ☧ians. To further speculate [bear with me and entertain me]: these seven antiphons may be related to a Hannukah service. Hannukah is celebrated over eight days, and its celebration moves about December, for it does not follow the solar calendar. The Hannukah story is found in the Old Testament books of the Maccabees, but is not canonical among the jews. It is possible, that, part of that celebration migrated from jewish to christian worship.

Now, for those who do not acknowledge, the canonicity of the books of the Maccabees, they refer to an intertestamentary period , wherein, those events occurred. The temples of Solomon, and the babylonian return [and later Herod] contained a menorah of seven branches. A miracle of the Maccabees was, that, a one day supply of lamp oil lasted eight. Now menorahs for Hannukah contain eight candles, plus one smaller lighting candle as a ninth branch.

O Oriens, splendor lucis æternæ, et sol justitiæ:
veni, et illumina sedentes in tenebris, et umbra mortis.

O Dayspring, splendor of Light eternal, and Sun of Justice:
come, and enlighten those who sit in darkness, and the shadow of death.

We see Jesus as the Messias, the☧, the True Light. The light of the menorah symbolises this, as do the candles lit during the dark of vespers.
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Glossary: Antiphon = an anthem, verse sung responsively
Breviary = a book containing the divine office, the daily canonical hours of prayer
Magnificat = the canticle, song, of Mary in Luke i. 46-55.
Vespers = the 6th of 7 hours of prayer, the evening hour, or vespertina synaxis
Vigil = the night before a feast, a period of watch and prayerful preparation
Maccabees = a family of 2nd century b. C. Juda, Seven martyrs commemorated on 1 August, 2 books of the Old Testament
Hannukah = dedication, consecration; the festival commemorating the events of 165 b.C.:

And they kept the dedication of the altar eight days, and they offered holocausts with joy, and sacrifices of salvation, and of praise. ― 1Maccabees iv. 56.
And Judas, and his brethren, and all the church of Israel decreed, that the day of the dedication of the altar should be kept in its season from year to year for eight days, from the five and twentieth day of the month of Casleu, with joy and gladness. ― 1Maccabees ⅳ. 59.

Menorah = candelabrum used in jewish worship