”The “O Antiphons” traditionally sung during the last week before Christmas (December 17-23), are more than a thousand years old. The traditional Advent hymn, “O Come, O Come, Emmanuel” Is probably the most popular and well known way of remembering some of those antiphons.
They are called the “O Antiphons” simply because each begins with the word, “O” Originally used in monastic vespers during the last seven days before Christmas; gradually they came into use in the whole church. The Gospel Acclamation at all Masses in these seven days uses the “O Antiphons” as well.
Each antiphon denotes an attribute of the coming Christ. In Latin, they are
December 17: O Sapientia (O Wisdom)
December 18: O Adonai (O Lord)
December 19: O Radix Jesse (O Root of Jesse)
December 20: O Clavis David (O Key of David)
December 21: O Oriens (O Dayspring)
December 22: O Rex Gentium (O King of the nations)
December 23: O Emmanuel (O With Us is God)