O Rex Gentium sets the Magnificat antiphon for December 22 - one of the great 'O' antiphons traditionally sung at Vespers in the week before Christmas. These texts are all invocations of the coming of Christ, each one offering praise to him using titles taken from the prophet Isaiah. This piece was commissioned in 2011 to form part of a complete set of the seven O Antiphons for the Merton College Choirbook. It is conceived in grand and declamatory style, owing a respectful debt to the great motets of Bruckner. Formed in A-B-A structure, it uses block harmony in the outer sections for impact, whilst the central part employs some more reflective and linear writing. A brief coda suggests an atmosphere of wonder and mystery at the prospect of Christ's birth.