I recommend the interesting website The Hymns and Carols of Christmas for a visit at this particular season of the Liturgical year and from that site take the texts given below:
O Virgo virginum, quomodo fiet istud? quia noc primam similem visa es, nec habere sequentem. Filæ Jerusalem, quid me admiramini? Divinum est mysterium hoc quod cernitis.
O Virgin of virgins! how shall this be? for never was there one like thee, nor will there ever be. Ye daughters of Jerusalem, why look ye wondering at me? What ye behold, is a divine mystery.
Dom Guéranger notes (see the link above) that this antiphon was used in the post-Tridentine Roman rite in Spain and some of her Dominions for the pro aliquibus locis feast of the Expectation of the BVM on 18th December. The old practice of having the antiphon on the 23rd seems to me at least an excellent one emphasising the inseparable link between the Mother of God and the mystery of the Incarnation. A beautiful acrostic was created (very common in the Sarum breviary) by taking the first letter of each antiphon in reverse order. So for the seven in the Roman Breviary this creates 'Ero Cras' that translates as 'Tomorrow, I will come'. With O Virgo virginum this becomes 'Vero Cras', 'Truly, tomorrow'.
There were also other 'Lesser O's' (again texts from the excellent site of Doug Anderson):
O Hierusalem, civitas Dei summi: leva in circuitu oculos tuos, et vide Dominum tuum, quia jam veniet solvere te a vinculis.
O Jerusalem, city of the great God: lift up thine eyes round about, and see thy Lord, for he is coming to loose thee from thy chains.
O Rex pacifice, Tu ante saecula nate: per auream egrede portam, redemptos tuos visita, et eso illuc revoca unde ruerunt per culpam.
O King of peace, that was born before all ages: come by the golden gate, visit them whom thou hast redeemed, and lead them back to the place whence they fell by sin.
O Gabriel, nuntius caelorum, qui clausis ianuis ad me introisti, et verbum annuntiasti: concipies et paries Emmanuel vocabitur.
O Gabriel! the messenger of heaven, who camest unto me through the closed doors, and didst announce the Word unto me : Thou shalt conceive and bear a Son, and he shall be called Emmanuel.
The last was generally replaced by O Thoma that I mentioned yesterday. Many years ago I speculated that it would be fascinating to interpolate all of the 'O's into the Magnficat on December 23rd as they all have the same tone, 2D. I then discovered that this had indeed been done in some Monastic establishment but didn't record my reference. A good friend I lunched with on my recent visit to Ireland suggested that it may have been Prinknash. I shall investigate this in due course.
In the Roman liturgy the day is of simple rite. The antiphons Rorate, caeli etc are sung at the second scheme of Lauds and at the Little Hours. At the Benedictus an antiphon special to this day is sung: Ecce completa sunt... Behold all things are accomplished... Ferial preces are sung, kneeling at Lauds and at the Little Hours.
At Mass, sung after None, the ministers wear folded chasubles and four candles are on the altar. The chants are ferial. The Mass is of the fourth Sunday of Advent but without the Alleluia and versicle after the Gradual. The second collect is of the BVM, Deus, qui de beate, and the third collect is for the Church, Ecclesiae. As always when violet vestments are worn Benedicamus Domino is the dismissal. As normal on 'kneeling days' the choir kneels for the orations and from the Sanctus to the Fraction.
At Vespers the antiphon O Emmanuel is sung, doubled, and with the choir standing. After the Magnificat and the repetition of the antiphon the ferial preces are sung, kneeling. Again at Compline preces are sung with the choir kneeling.
In the 'ancient' liturgical books of 1962 there are no ferial preces. Antiphons are of course 'doubled' regardless of the rank of day, at Prime the festal Regi saeculorum is sung rather than the ferial Pacem. At Mass, sung after Terce (in the 'ancient' rite of 1962) the distinction of singing the Mass after different Hours depending on the rank of day is lost and all Masses are sung after Terce except for a 'grave reason'), the ministers wear dalmatic and tunicle, there is no collect of the BVM or for the Church and Ite, missa est is the dismissal. At Vespers the significance of the doubled 'Great 'O' antiphon is lost and there are no preces at either Vespers or Compline.