The "O Antiphons" are a traditional Advent devotion dating to as early as the 5th century. Benedictine monasteries provide some of the earliest evidence of their usage and by the 8th century they were commonly used in Roman churches. There are seven "O's" (the name simply derives from the start of each antiphon -- "O Sapienta, O Adonai, etc.). Thus the first is recited on December 17th and the last on December 23rd.
Each antiphon is based on a title used for Jesus in Scripture thus (and I realize I've used the word "thus" in consecutive sentences):
- 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 God-With-Us)
In Anglican usage the O Antiphons are traditionally used before and after the Magnificat at Evening Prayer (sung at Evensong) or as the Alleluia verse before the gospel reading during the appointed days. For many, they are used as private devotions during the waning days of Advent.
Click here to access a full list of the O Antiphons (along with their Scriptural citations) in both Latin and English. This comes from the Liturgy blog of the wonderful New Zealander Bosco Peters. Enjoy.