For example, O.C.D does not designate a nun with obsessive compulsive disorder.
In the Catholic Church, members of religious orders, as well as those entrusted with various ecclesiastical duties, may sign their name with an abbreviation. However, many people have no idea what the abbreviation stands for or what it means (and sometimes they confuse them with other abbreviations).
Here is a short slideshow with some of the most common abbreviations for Catholic religious and clergy members and what they mean.
Here are 10 official titles of Pope Francis and their meaning
Reading Christian art: 5 Common abbreviations and what they mean