Sister Margaret first learned about the Dominican rule from Pere C. B. Moulaert, a Belgian Fathers of the Order, with whom she had become acquainted during the time of her lay association with the Order at Bruges. Pere Moulaert was the author of a Manual of the Brothers and Sisters of the Third Order. This was an annotated reprinting of the treatise of the Rev. Pere J. B. Feuillet published in 1845. Drawing upon this manual Sister Margaret wrote the “Rules and Constitutions, or Customs for the present regulation of the Third Order of our Holy Father St Dominic: under the special protection of the Ever-Blessed Virgin and St. Catherine of Sienna.” The Dominican initials OP after a Sister's name now stand for Order of Preachers, but originally they stood for Order of Penance. Dominican Sisters used the initials OSD for Order of Saint Dominic until the early 20th century.
Drane, Augusta Theodosia (Mother Francis Raphael), Life of Mother Margaret Mary Hallahan: Foundress of the English Congregation of St. Catherine of Siena of the Third Order of St. Dominic, Longmans, Green and Co., New York, New York, 1929, p. 99.