A little more about the O Antiphons

From the Catholic Encyclopedia:

The seven antiphons to the Magnificat in the ferial Office of the seven days preceeding the vigil of Christmas; so called because all begin with the interjection "O". Their opening words are: (1) "O Sapientia", (2) "O Adonai", (3) "O Radix Jesse", (4) "O Clavis David", (5) "O Oriens", (6) "O Rex Gentium", (7) "O Emmanuel".

Addressed to Christ under one or other of His Scriptural titles, they conclude with a distinct petition to the coming Lord (e. g.: "O Wisdom … come and teach us the way of prudence"; "O Adonai … come and redeem us by thy outstretched arm"; "O Key of David … come and lead from prison the captive sitting in darkness and in the shadow of death" etc.).

Couched in a poetic and Scriptural phraseology they constitute a notable feature of the Advent Offices. These seven antiphons are found in the Roman Breviary; but other medieval Breviaries added (1) "O virgo virginum quomodo fiet" (2) "O Gabriel, nuntius cœlorum", subsequently replaced, almost universally, by the thirteenth-century antiphon, "O Thoma Didyme", for the feast of the Apostle St. Thomas (21 December).

Some medieval churches had twelve greater antiphons, adding to the above (1) "O Rex Pacifice", (2) "O Mundi Domina", (3) "O Hierusalem", addressed respectively to Our Lord, Our Lady, and Jerusalem.

The Parisian Rite added two antiphons ("O sancte sanctorum" and "O pastor Israel") to the seven of the Roman Rite and began the recitation of the nine on the 15th of December.

Prose renderings of the Roman Breviary O's will be found in the Marquess of Bute's translation of the Roman Breviary (winter volume). Guéranger remarks that the antiphons were appropriately assigned to the Vesper Hour because the Saviour came in the evening hour of the world (vergente mundi vespere, as the Church sings) and that they were attached to the Magnificat to honour her through whom He came.

[The following isn't from the Catholic Encyclopedia, but of interest:] Some surmise that the Benedictine monks arranged these antiphons with a definite purpose. If one starts with the last title and takes the first letter of each of the O Antiphons - Emmanuel, Rex, Oriens, Clavis, Radix, Adonai, Sapientia - the Latin words ero cras are formed, meaning, “Tomorrow, I will come.”