The Divine Office and the Mass, while both Liturgy, use the term in different ways. While Mass is Liturgy, it is not merely liturgy (public work), but purely divine gift. As such it is raised above the Divine Office. Let’s examine the two.
When we speak about the Divine Office, we are using the original Latin term of ‘Office’, which is a composite of ‘Opus’ and ‘Facere’, literally translated as ‘to do work’. Despite it’s similarities with ‘Liturgy’ it is always used within the context of a act motivated by a being’s own will. It is liturgy in the sense that it is a public work of the Church, but unlike sacramental liturgy like the Mass, it is solely the work of man (the Church) in response to God. It is a human working, but it’s ultimate aim is God. It find’s its source in God’s call and finds its end in closer union to God. Hence ‘Divine Office’. In this sense, it is not so much the work of God but rather man’s work concerning God. It is the highest form of Liturgy which we, as humans, can work upon our own power. Thus, it is our work of God.
The Mass is the ultimate form of the Liturgy. It is Liturgy, not in the sense that man is doing the public work, but rather that God is the one doing the action. In the Liturgy of the Word, the Father makes himself known through the Son, the Word, evident in the scriptures and the people receive that Word through the inspiration of the Holy Spirit. In the Liturgy of the Eucharist, the Father works transubstantiation through the Holy Spirit to make His Son physically present in the Eucharist: Body, Blood, Soul, and Divinity. Man cannot enact the Mass without God. Reading the Scriptures would just be words on a page. The Liturgy of the Eucharist would simply be repeating a formula and passing out bread and wine.
To sum up, the Divine Office is man’s work of God. The Mass is so much more.
Hope this helped.
Br. Ben, CRM