The O Antiphons, also known as the great Os, are Magnificat antiphons used at Vespers of the last seven days of Advent in Western Christian traditions. They are also used as the Alleluia verses on the same days in the post-1970 form of the Catholic Mass, although the Lectionary moves O Emmanuel to the 21st, uses Rex Gentium on both the 22nd and 23rd, and places O Oriens on the morning of the 24th, with authorization for the traditional ordering from the 17th through the 23rd.
December 17 A Child at Rest Isaiah 11:6
December 18 The Power of Children Ps 18:28-29