Offertory Song: Music used during the procession of gifts to the celebrant and as the altar is prepared.

Opening prayer: This prayer by the celebrant expresses the general theme of the celebration.

Opus Dei: A personal prelature dedicated to spreading through society an awareness of the call to Christian virtue, awareness, and witness in one's life and work. Members are not of a religious order, do not take vows, but sometimes live in community.

Ordain - Ordination: The proper terms in Catholic usage for references to the conferral of the sacrament of holy orders on a deacon, priest or bishop.

Order, Congregation, Society: Religious orders is a title loosely applied to all religious groups of men and women. A society is a body of clerics, regular or secular, organized the purpose of performing an apostolic work. Congregation is any group bound together by common rules.

Ordinary: Diocesan bishops, religious superiors, and certain other diocesan authorities with jurisdiction over the clergy in a specific geographical area, or the members of a religious order.

Ordination/Ordain: The sacramental rite by which a "sacred order" is conferred (diaconate, priesthood, episcopacy).