How to remember the order of the O-Antiphons?
Well, in reverse order, the initials of the second word in each compose the phrase reverently put on the lips of Our Lord, the King Who is to come: Ero cras - "I shall be (with you) to-morrow". Holy Church, His Bride, therefore cries out, responding to His Advent in wondering joy, naming him with blessed titles drawn from Holy Writ, inspired by the Holy Ghost.
The Antiphons are therefore: O Sapientia, O Adonai, O Radix Jesse, O Clavis David, O Oriens, O Rex Gentium, O Emmanuel.
So to-night comes O Adonai: for the text, the chant and the sources, just follow this link to my post concerning all this last year...
(Obviously I refer to the Roman Rite here; the English mediæval rites began these anthems a day earlier, and had an extra one, O Virgo Virginum; and other mediæval uses had as many as twelve of them.)