The O Antiphons for Advent begin tomorrow and will continue through December 23. The particular antiphon for the day can be found in two places: It is part of the Alleluia verse for the liturgy of the day; and in the evening; it's placed before and after the Magnificat at evening prayer for the Liturgy of the Hours.
The Advent antiphons are called the "O Antiphons" because each day's antiphon begins with an "O" ... O Wisdom, O Key of David, O Flower of Jesse's stem...
- Little Blue Book, Diocese of Saginaw