“The O Antiphons (also known as the Great Advent Antiphons or Great Os) are Magnificat antiphons used at Vespers on the last seven days of Advent in Western Christian traditions. They likely date to sixth-century Italy, when Boethius refers to the text in The Consolation of Philosophy. They subsequently became one of the key musical features of the days leading up to Christmas.

The "O" Antiphon for December 21 is "O Oriens" (O Morning Star)