The “O" Antiphons refer to the seven antiphons that are recited (or chanted) on each day of the special period of Advent preparation known as the Octave before Christmas. This period runs from December 17 through December 23.
The exact origin of the “O" Antiphons is not known but written references about the antiphons in the late fifth and early sixth centuries suggest their presence at that time. By the eighth century, they were in use in the liturgical celebrations in Rome. 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.
The importance of “O" Antiphons is twofold:
The symbols for the seven "O" Antiphons are shown at the left. For information on the prayer for each antiphon and the related scripture from Isaiah look at the
"O" Antiphons handout. More information available via the references below.
Wikipedia
http://en.wikipedia.org/wiki/O_Antiphons
Catholic Online
http://www.catholic.org/clife/advent/advent.php?id=7
Catholic Education Resource Center
http://www.catholiceducation.org/en/culture/catholic-contributions/what-are-the.html
Catholic Culture
http://www.catholiceducation.org/en/culture/catholic-contributions/what-are-the.html