The O antiphons are verses of praise used during the last seven days of Advent (December 17-23). They are referred to as the "O Antiphons'' because the title of each one begins with the "O" addressing a given title in the verse to Christ. Each antiphon is a reflection on the name of Christ, and praise of one of his attributes mentioned in Scripture.
December 23rd - O Emmanuel - O God With Us
December 22nd - O Rex Gentium - O King of the Nations
December 21st - O Oriens - O Dayspring
December 20th - O Clavis David - O Key of David
December 19th - O Radix Jesse - O Root of Jesse
December 18th - O Adonai - O Lord of Might
December 17th O Sapientia – O Wisdom