The Great “O” Antiphons were sung before and after the Psalm at Vespers during the last seven days of Advent. They were used to create the hymn “O Come, O Come, Emmanuel,” which is the Hymn of the Day for the Fourth Sunday in Advent: Rorate Coeli. The “O” Antiphon for December 20 is:
O Key of David and scepter of the house of Israel, You open and no one can close, You close and no one can open:
Come and rescue the prisoners who are in darkness and the shadow of death.