From an article at Catholic Culture:
As Elsa Chaney in Twelve Days of Christmas states, "[The O antiphons] seem to sum up all our Advent longing as they paint in vivid terms the wretched condition of mankind and his need of a Savior.'"
The "O" Antiphons are the verses for the ancient hymn O Come, O Come Emmanuel. The first letter of the Messianic titles: Emmanuel, Rex, Oriens, Clavis, Radix, Adonai, Sapientia—spell out Latin words ERO CRAS, meaning, "Tomorrow, I will come."
The antiphons are part of the evening prayer of the Divine Office, the antiphon before and after the Magnificat. They are also the alleluia verse before the Gospel at Mass.
In these last few days before Christmas, join the Church in praying the O Antiphons.