source: Hymns and Carols of Christmas gives us this information about the O Antiphons:
“This is one of the oldest of Christian prayers — referred to as the “O” Antiphons, the “Greater” Antiphons, and “The Seven O’s.”
These seven antiphons were recited as a part of the evening Vespers prayers of the Catholic Church before and after The Magnificat in the Octave before Christmas, December 17 to 23 (the Vespers for Dec. 24, Christmas Eve, are those for the Christmas Vigil). Prior to the Reformation, it was sung from 16 to 23 December, omitting St. Thomas’ Day (December 21). These seven days are also known as the Greater Ferias.
Each of the seven stanzas addressed the Messiah by one of his titles, each one praising the coming of the Savior by a different name, and closing with petitions appropriate to the title. Thus:
O EMMANUEL, God with us,
Our King and Lawgiver, the expected of the nations and their Saviour:
COME to save us, O Lord our God. Amen.
One verse was sung or chanted each evening (as opposed to being sung together as a single hymn, as we do today).
According to one source, on December 17th the Abbot would intone the first Antiphon, O Sapientia. On successive nights, each principal officer of the monastery would take his turn with another of the Antiphons. A After the service, the officer was expected to provide some sort of treat, usually edible, for all the monks.
The antiphons date back at least to the reign of Charlemagne (771-814), and the 439 lines of the English poem Christ,by Cynewulf (c. 800), are described as a loose translation and elaboration of the Antiphons. B One source stated that Boethius (c. 480-524) made a slight reference to them, thereby suggesting their presence at that time. C Julian reports that two 11th century copies can be found in manuscripts in the British Museum and the Bodleian. The usage of the “O Antiphons” was so prevalent in monasteries that the phrases, “Keep your O” and “The Great O Antiphons” were common parlance.”