Founded in 1928 by Josemaría Escrivá (now a Saint within the Catholic church), it has received criticism as being secretive, cladestine and cultish. Opus Dei is a very conservative and orthodox flavour of Catholicism, and the organization also supported the fascist Franco regime in Spain.
Wiki has this to say about Opus Dei...
Opus Dei is an organization of the Catholic Church. As such, it shares the theology of the Catholic Church. Opus Dei places special emphasis on certain aspects of Catholic doctrine. A central feature of Opus Dei's theology is its focus on the lives of the ordinary Catholics who are neither priests nor monks. Opus Dei emphasizes the "universal call to holiness": the belief that everyone should aspire to be a saint, that sanctity is within the reach of everyone, not just a few special individuals.
Opus Dei does not have monks or nuns, and only a minority of its members are part of the priesthood. A related characteristic is Opus Dei's emphasis on uniting spiritual life with professional, social, and family life. Whereas the members of some religious orders might live in monasteries and devote their lives exclusively to prayer and study, members of Opus Dei lead ordinary lives, with traditional families and secular careers, and strive to "sanctify ordinary life". Indeed, Pope John Paul II called Escrivá "the saint of ordinary life".