O Adonai, et Dux domus Israel, qui Moysi in igne flammae rubi apparuisti, et ei in Sina legem dedisti: veni ad redimendum nos in brachio extento.
O Adonai, and Ruler of the house of Israel, who appeared unto Moses in the burning bush, and gave him the Law on Sinai, come to redeem us with arm outstretched!
Here are the corresponding modified hymn verses from "Veni, Veni, Emmanuel" and "O Come, O Come Emmanuel."
Veni, veni, Adonai, Qui populo in Sion, Legem dedisti vertice. In majestate gloriae. R: Gaude! Gaude! Emmanuel, nascetur pro te Israel!
Come, O come, O Adonai [Lord], Who to thy people on Sinai’s summit didst give the law in glorious majesty. R. Rejoice! Rejoice! Emmauel has come to thee, O Israel.
Here is Dom Gueranger’s commentary about this O antiphon for December 18.
O Sovereign Lord! O Adonaï! come and redeem us, not by thy power, but by thy humility. Heretofore, thou didst show thyself to Moses thy servant in the midst of a mysterious flame; thou didst give thy law to thy people amidst thunder and lightning; now, on the contrary, thou comest not to terrify, but to save us. Thy chaste Mother having heard the Emperor's edict, which obliges her and Joseph her Spouse to repair to Bethlehem, she prepares everything needed for thy divine Birth. She prepares for thee, O Sun of Justice! the humble swathing-bands, wherewith to cover thy nakedness, and protect thee, the Creator of the world, from the cold of that mid-night hour of thy Nativity! Thus it is that thou willest to deliver us from the slavery of our pride, and show man that thy divine arm is never stronger than when he thinks it powerless and still. Everything is prepared, then, dear Jesus! thy swathing-bands are ready for thy infant limbs! Come to Bethlehem, and redeem us from the hands of our enemies.
Here is a link from New Liturgical Movement that includes a link to a sound file of the chant.
History and Mystery: The O Antiphons in a Favorite Hymn" for my post on the relationship between the O antiphons,"Veni, Veni, Emmanuel" and "O Come, O Come Emmanuel."