The antiphon for 19th December is O Radix Iesse (O Root of Jesse):
O Root of Jesse, which standest for an ensign of the people, at Whom the kings shall shut their mouths, Whom the Gentiles shall seek, come to deliver us, do not tarry.
O Radix Iesse, qui stas in signum populorum, super quem continebunt reges os suum, quem Gentes deprecabuntur: veni ad liberandum nos, jam noli tardare.
Click here to listen to the chant.
All of the O antiphons are prayed before and after the Magnificat during Vespers. The Magnificat, also called the Cantical of Mary, is the great prayer of Our Lady from Luke 1:46-55. Each night at Vespers, or Evening Prayer, which is the penultimate hour of the liturgy of the hours, the antiphon for the day is recited, then the Magnificat, and then the antiphon is repeated. You can hear (and see) it chanted in Latin by the Dominicans here with today’s antiphon. The English words are below.
My soul proclaims the greatness of the Lord,
my spirit rejoices in God my Savior
for he has looked with favor on his lowly servant.
From this day all generations will call me blessed:
the Almighty has done great things for me,
and holy is his Name.
He has mercy on those who fear him
in every generation.
He has shown the strength of his arm,
he has scattered the proud in their conceit.
He has cast down the mighty from their thrones,
and has lifted up the lowly.
He has filled the hungry with good things,
and the rich he has sent away empty.
He has come to the help of his servant Israel
for he remembered his promise of mercy,
the promise he made to our fathers,
to Abraham and his children forever.