Some of the most common ones:
C.P. - Congregation of the Passion (Passionists)
C.S.P. - Congregation of St. Paul (Paulists)
C.Ss.R - Congregation of the Most Holy Redeemer (Redemptorists)
M.C. - Missionaries of Charity (founded by Mother Teresa of Calcutta)
O.Carm. - Order of Our Lady of Mt. Carmel (Carmelites)
O.C.D. - Order of Our Lady of Mt. Carmel, Discalced (Carmelites under the reformed rule of Sts. Teresa & John of the Cross)
O.C.S.O, O.C.R. - Order of Cistercians of the Strict Observance (Trappists)
O.Cist. - Cistercians of the Ancient Observance (Cistercians)
O.F.M - Order of Friars Minor (Franciscans)
O.F.M. Cap - Order of Friars Minor Capuchin (Capuchins)
O.H. - Order of Hospitalers (Hospitalers)
O.P. - Order of Preachers (Dominicans)
O.Praem. - Order of the Canons Regular of Prémontré (Norbertines)
O.S.A. - Order of St. Augustine (Augustinians)
O.S.B. - Order of St. Benedict (Benedictines)
O.S.B.Cam. - Camaldolese Benedictines (Camaldolese)
O.S.C. - Order of St. Clare (Poor Clares)
O.S.M. - Order of Friars, Servants of Mary (Servites)
O.Ss.S. - Order of St. Savior (Brigittine)
S.J. - Society of Jesus (Jesuits)
more initials listed here
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C.P. - Congregation of the Passion (Passionists)
C.S.P. - Congregation of St. Paul (Paulists)
C.Ss.R - Congregation of the Most Holy Redeemer (Redemptorists)
M.C. - Missionaries of Charity (founded by Mother Teresa of Calcutta)
O.Carm. - Order of Our Lady of Mt. Carmel (Carmelites)
O.C.D. - Order of Our Lady of Mt. Carmel, Discalced (Carmelites under the reformed rule of Sts. Teresa & John of the Cross)
O.C.S.O, O.C.R. - Order of Cistercians of the Strict Observance (Trappists)
O.Cist. - Cistercians of the Ancient Observance (Cistercians)
O.F.M - Order of Friars Minor (Franciscans)
O.F.M. Cap - Order of Friars Minor Capuchin (Capuchins)
O.H. - Order of Hospitalers (Hospitalers)
O.P. - Order of Preachers (Dominicans)
O.Praem. - Order of the Canons Regular of Prémontré (Norbertines)
O.S.A. - Order of St. Augustine (Augustinians)
O.S.B. - Order of St. Benedict (Benedictines)
O.S.B.Cam. - Camaldolese Benedictines (Camaldolese)
O.S.C. - Order of St. Clare (Poor Clares)
O.S.M. - Order of Friars, Servants of Mary (Servites)
O.Ss.S. - Order of St. Savior (Brigittine)
S.J. - Society of Jesus (Jesuits)
more initials listed here
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