This is the third of the Great Antiphons/O Antiphons sung at the Magnificat at Vespers during the last days of Advent. This antiphon is traditionally used on December 19. (In the Paris Breviary it is used on December 17 and in medieval English usage it is on December 18.)
O Radix Iesse, qui stas in signum populorum, super quem continebunt reges os suum, quem Gentes deprecabuntur: veni ad liberandum nos, iam noli tardare.
Words: Anonymous, ca. 9th C.
Tune: “O Radix Iesse” Gregorian Chant, Mode II, traditional.
This is the English translation of the text as it appears in Fr. Matthew Britt’s 1922 text of The Hymns of the Breviary and Missal:
O Root of Jesus, who standest as the ensign of the people, before whom kings shall not open their lips; to whom the Gentiles shall pray: come and deliver us, tarry now no more.