The antiphon for 18th December is O Adonai, addressing Jesus under the title of Lord and God of Israel.
Adonai, and Ruler of the house of Israel, Who didst appear unto Moses in the burning bush, and gavest him the law in Sinai, come to redeem us with an outstretched arm.
O Adonai, et Dux domus Israel, qui Moysi in igne flammæ rubi apparuisti, et ei in Sina legem dedisti: veni ad redimendum nos in brachio extento.
Click here to hear it sung by the Domincan students at Blackfriars.
Even if you have not heard of the O Antiphons before, you may well know the hymn O Come, O Come Emmanuel, which is based upon them. This link shows the overlap of the traditional antiphons with the verses of the hymn (in English and Latin), as well as giving the scriptural references that are their origin.