Today’s feast (18/12) is a very old feast in the Iberian peninsula; it is still commemorated in the cathedral of Evora in the novus ordo, but doesn’t seem to be celebrated elsewhere in Portugal. The Bragan missal, however, does keep the feast.
The Mass is identical to that of the Annunciation apart from the following differences:
Introit: Isa 45:8; V./ Ps 18:2
Alleluia: Lk 1:31
Preface: of the Blessed Virgin with the inclusion of “et te in Expectatione”
Our Lady of the O
While not present in the ’62 edition, one can find this Mass in earlier editions, in the section for aliquibus in locis celebrandae.
Alma Bracarense 
É (ou devia ser) celebrada nas igrejas e capelas que têm como patrona Nossa Senhora do Ó (ou da Expectação).