The O Antiphons comprise a liturgical chain of ancient Latin Christian poetry. Their purpose is to tell us who is incarnate as we prepare for the feast of Christ’s birth. They are rich with the messianic imagery of the scriptures, each structured around a title of Christ, respectively, Wisdom, Adonai, Root of Jesse, Key of David, Rising Dawn, King of Nations, and Emmanuel.
We do not know exactly how old these antiphons are, but the evidence suggests sixth century at the latest, and quite possibly earlier. In the Western Church, they are still used at vespers with the Magnificat for the last seven days of Advent (December 17-23, though many Anglicans follow the English tradition of using an eighth antiphon to the Virgin on the 23rd, and thus start a day early). The Christmas hymn, O Come, O Come Emmanuel is an adaptation of the whole set.
I believe these antiphons present us with a complete orthodox Christology. I believe they also image a kind of cosmic descent, illuminating various aspects of the mystery of the incarnation. Meditating on the O Antiphons may be helpful in preparing to celebrate the coming of our Lord.