The carol 'O Come, O Come, Emmanuel' is based on which ancient liturgical texts?

“O Come, O Come, Emmanuel” is based on the Great O Antiphons, ancient liturgical texts used during Advent.

The antiphons are seven short Latin chants sung or recited at Vespers before Christmas, traditionally from December 17 through December 23. Each begins with “O” and invokes Christ under a biblical title, including Wisdom, Adonai, Root of Jesse, Key of David, Dayspring, King of Nations, and Emmanuel.

The hymn is not itself an unchanged ancient chant. Its Latin metrical text, “Veni, Veni, Emmanuel,” was first documented in print in the 1710 edition of Psalteriolum Cantionum Catholicarum. The familiar English translation was produced by John Mason Neale in the nineteenth century, especially in 1851.

A related curiosity is the acrostic formed by the titles in reverse order: “Ero cras,” usually translated as “Tomorrow, I will come.” The hymn’s title comes from the final antiphon, “O Emmanuel,” meaning “O God with us.”