O Salutaris Hostia is from the last two verses of Verbum Supernum, one of the five Eucharistic Hymns written by St. Thomas Aquinas (1225-1274) at the request of Pope Urban IV (1261-1264) when the Pope first instituted the Feast of Corpus Christi in 1264. The prayer is still used today, often at exposition of the Blessed Sacrament.

O SALUTARIS Hostia
Quae coeli pandis ostium.
Bella premunt hostilia;
Da robur, fer auxilium.

O SAVING Victim opening wide
The gate of heaven to all below.
Our foes press on from every side;
Thine aid supply, Thy strength bestow.

Uni trinoque Domino
Sit sempiterna gloria:
Qui vitam sine termino,
Nobis donet in patria.

Amen.

To Thy great name be endless praise
Immortal Godhead, One in Three;
Oh, grant us endless length of days,
In our true native land with Thee.

Amen. (tr. E Caswall)