THE GREAT ANTIPHONS or the O ANTIPHONS are used from December 17-23 to recall the fulfillment of all Old Testament hopes in Jesus Christ. In fact, you already know them!, since they are also in our favorite Advent hymn, "O Come, O Come, Emmanuel"
See the following link for more information on them:
And some good reflections on each antiphon specifically:
The traditional antiphon text (from the Graduale Simplex / Liber Usualis) with the chant music and audio: