Why, again, is the Expectation of the Blessed Virgin usually called Our Lady of the O? The reason behind this is what the first 14 seconds of the video below reimagine:
This is the so-called eighth Great Antiphon. In the Roman Rite, we have seven O antiphons sung at vespers in the last seven days before Christmas. When chronologically read, the first letters of the invocations spell SARCORE, which, when reversed, yields, ERO CRAS, Latin for the English Tomorrow, I shall be. The eighth O antiphon, however, is about the Blessed Virgin, and it is sung on the feast of the Expectation.
At the second vespers of this feast, on 18 December, in the Cathedral of Toledo, the interjection O at the beginning of the antiphon at the Magnificat used to be intoned by all clergy in attendance, with neither order nor arrangement, being prolonged in order to express the yearning of the world for the coming of the Messiah. (More details and readings here.) The reimagining above is neither sufficiently prolonged nor candidly disorganised, but it captures the spirit.
Ut in omnibus laudetur Dominus.