The O Antiphons, those in the Roman Catholic tradition that I am familiar with, are sung words using images from the OT and recited at Vespers from December 17 to December 23 inclusive. They are the introduction to Mary’s prayer which we now sing daily in our evening prayer service. I think the Church of England has the same tradition; they have traditionally been used as antiphons to the Magnificat at Evening Prayer. I think the first letters of the titles taken backwards form a Latin acrostic of "Ero Cras" which translates in common language to "I will be there tomorrow", which mirrors the theme of the antiphons. It’s also seen as a response of Christ himself to the heartfelt prayer of his people: "Tomorrow I will be there". The idea of them is possibly that we have prepared for the Advent of Christ with these seven Messianic titles, and now we are preparing for Christ to come. "Tomorrow, I will come." God has seen our sin and shame, he has heard our cries and condemnations, our trials and tortures and answers them with the promise, and I will be there very soon.