“O come, O come, Emmanuel” (Latin: “Veni, veni, Emmanuel”) is a Christian hymn for Advent, which is also often published in books of Christmas carols.[1][2][3] The text was originally written in Latin. It is a metrical paraphrase of the O Antiphons, a series of plainchant antiphons attached to the Magnificat at Vespers over the final days before Christmas. The hymn has its origins over 1,200 years ago in monastic life in the 8th or 9th century. Seven days before Christmas Eve monasteries would sing the “O antiphons” in anticipation of Christmas Eve when the eighth antiphon, “O Virgo virginum” (“O Virgin of virgins”) would be sung before and after Mary’s canticle, the Magnificat (Luke 1:46b–55). The Latin metrical form of the hymn was composed as early as the 12th century.[4]

The 1851 translation by John Mason Neale from Hymns Ancient and Modern is the most prominent by far in the English-speaking world, but other English translations also exist. Translations into other modern languages (particularly German) are also in widespread use. While the text may be used with many metrical hymn tunes, it was first combined with its most famous tune, often itself called Veni Emmanuel, in the English-language Hymnal Noted in 1851. Later, the same tune was used with versions of “O come, O come, Emmanuel” in other languages, including Latin.

Each stanza of the hymn consists of a four-line verse (in 88.88 meter with an AABB rhyme scheme), paraphrasing one of the O Antiphons. There is also a new two-line refrain (again in 88 meter): “Gaude, gaude! Emmanuel / nascetur pro te, Israel”, i.e., “Rejoice, Rejoice! Emmanuel will be born for you, O Israel”. There are only five verses: two of the antiphons are omitted and the order of the remaining verses differs from that of the O Antiphons, most notably the last antiphon (“O Emmanuel”) becomes the first verse of the hymn and gives the hymn its title of “Veni, veni, Emmanuel”:

Source: Wikipedia

The Roman Church has been singing the “O” Antiphons since at least the eighth century. They are the antiphons that accompany the Magnificat canticle of Evening Prayer from December 17-23. They are a magnificent theology that uses ancient biblical imagery drawn from the messianic hopes of the Old Testament to proclaim the coming Christ as the fulfillment not only of Old Testament hopes, but present ones as well. Their repeated use of the imperative “Come!” embodies the longing of all for the Divine Messiah.

Source: From the United States Conference of Catholic Bishops https://www.usccb.org/prayers/o-antiphons-advent