The Roman Church has been singing the "O" Antiphons since at least the eighth century. They are the antiphons that accompany the Magnificat canticle of Evening Prayer from December 17-23. They are a magnificent theology that uses ancient biblical imagery drawn from the messianic hopes of the Old Testament to proclaim the coming Christ as the fulfillment not only of Old Testament hopes, but present ones as well. Their repeated use of the imperative "Come!" embodies the longing of all for the Divine Messiah.
USCCB on the O Antiphons