Here's the entry on O Antiphons from Fr. Richard McBrien's* Encyclopedia of Catholicism:

A series of seven antiphons, each beginning with the invocative "O," appointed for the Magnificat at Vespers celebrated between December 17 and 23. The texts are a mosaic of biblical verses from prophetic and wisdom books. Already known to Amalar of Metz (died circa 850), both texts and music probably date from the eighth or even the seventh centuries.
The texts are:
• O Sapientia (Wisdom) (December 17)
• O Adonai (Lord) (December 18)
• O Radix Jesse (Root of Jesse) (December 19)
• O Clavis David (Key of David) (December 20)
• O Oriens (Radiant Dawn) (December 21)
• O Rex Gentium (King of All Nations) (December 22)
• O Emmanuel (literally, God-with-us) (December 23)

Here's McBrien's definition of Vespers:

(Latin, "evening star"), or Evening Prayer, the liturgy of the Church celebrated in the early evening as daylight ends. One of the two principle Hours of prayer, Vespers includes a hymn, two psalms and a New Testament canticle, a reading from Scripture (usually the Pauline Letters), a proper responsory, the Magnificat with its antiphon, a litany of intercessory prayer, the Lord's Prayer, a concluding blessing, and dismissal.

Vespers is part of what is now called the Liturgy of the Hours:

The public prayer of the Church for praising God and sanctifying the day. It is also known as the Divine Office. It consists of an Office of Readings, Morning and Evening Prayer, Daytime Prayer, and Night Prayer.

The Liturgy of the Hours is printed in the Breviary:

...The breviary represents a late medieval compilation of several books: the antiphonary or book of short verses (antiphons), psalter or book of psalms, lectionary or book of lessons, martyrology or book of martyrs, and hymnary or book of hymns. It was first fashioned during the eleventh and twelfth centuries to assist the daily prayer of the mendicant orders, whose members could not have been expected to carry such a library on their travels. The breviary contributed to the privatization and clericalization of the Church's daily prayer. It came to be regarded as a book for priests, with the Liturgy of the Hours only for clerics.

From the third century, Christians gathered for prayer in the morning and evening, supplemented by private prayer at rising, at the third, sixth, and ninth hours, upon retiring, and (interrupting sleep) during the night. During the Middle Ages, the number of hours became set at the following: Vespers, Compline, Matins, Lauds, Prime (1), Terce (3), Sext (6), and None (9 - pronounced "known").

I was a student in a Catholic seminary in Milwaukee from 1962-70. In the early years, we'd see our professors walking the halls and the grounds, reading their breviaries in Latin. Priests were required to pray the Divine Office daily, and it took them an hour or two per day. Monks and some nuns recited or chanted the Office together, often in antiphonal style with the the two sides of the chapel chanting alternate lines, back and forth - it really was wonderful to hear this. In my last two years of the seminary (I left 4 years before my remaining classmates were ordained priests), we students prayed morning and evening prayers in this style. There really was something wonderful about it, but I'm glad we didn't do the whole thing. We spent maybe ten minutes in the morning and fifteen at night.

To warm up before evening prayer, we'd gather every evening in the vestibule of the chapel, smoke cigarettes, and sing Engerbert Humperdinck songs. I suppose "Last Waltz" was the most popular pre-prayer song.

In most Catholic parishes nowadays, the Liturgy of the Hours is celebrated only on special occasions, and many priests no longer recite the Divine Office when they aren't praying with others.

-Joe Offer-

*McBrien is considered to be a liberal. Some right-wing Catholics call him a heretic. To me, that means he must be a reasonably credible source.