The “O Antiphons” are antiphons (a short prayer, Scripture excerpt, etc) used before the Magnificat (the song of Mary from Luke 1:46-55) at Vespers (Evening Prayer) during the last seven days of Advent in the Catholic Church.
They are called the “O Antiphons” because the title of each begins with the address “O”. Each antiphon is a title of Christ, derived from attributes in Scripture. They are:
December 17: O Sapientia (Wisdom)
December 18: O Adonai (Lord)
December 19: O Radix Jesse (Root of Jesse)
December 20: O Clavis David (Key of David)
December 21: O Oriens (Dayspring)
December 22: O Rex Gentium (King of the nations)
December 23: O Emmanuel (With Us is God)
In the Catholic Church, they are used during Evening Prayer from December 17 to December 23 inclusive.
If you are interested in these Antiphons, we have them all listed on our Advent Prayers page.