Here is the O Antiphon for today:
Dec. 19 - O Radix Jesse: “O Flower of Jesse’s stem, you have been raised up as a sign for all peoples; kings stand silent in your presence; the nations bow down in worship before you. Come, let nothing keep you from coming to our aid.” (cf. Isa 11:1, 10)
Also see my previous post, "What Are the O Antiphons?"