Opus Dei is a real group, while Priory of Sion is fictitious. (Or at least, the real group had a fictitious history.)
"Opus Dei, formally known as The Prelature of the Holy Cross and Opus Dei, is an organization of the Roman Catholic Church that teaches the Catholic belief that everyone is called to holiness and that ordinary life is a path to sanctity. The majority of its membership are lay people, with secular priests under the governance of a prelate (bishop) appointed by the pope. Opus Dei is Latin for 'Work of God', hence the organization is often referred to by members and supporters as 'the Work'. ... Controversies about it have centered around criticisms of its alleged secretiveness, its recruiting methods, the alleged strict rules governing members, its acknowledged practice by celibate members of mortification of the flesh, its alleged elitism and misogyny, the right-leaning politics of most of its members, and the alleged participation by some in authoritarian or extreme right-wing governments, especially the Francoist Government of Spain until 1978. ... In recent years, Opus Dei has received international attention due to the novel The Da Vinci Code and its film version of 2006. ... In The Da Vinci Code, Opus Dei is portrayed as a Catholic organization that is led into a sinister international conspiracy. ... In general, The Da Vinci Code has been sharply criticized for its numerous factual inaccuracies, and its theory has been debunked by a wide array of scholars and historians."
https://en.wikipedia.org/wiki/Opus_dei
"Priory of Sion is a name given to multiple groups, both real and fictitious. The most notorious is a fringe fraternal organization, founded and dissolved in France in 1956 by Pierre Plantard. In the 1960s, Plantard created a fictitious history for that organization, describing it as a secret society founded in the Kingdom of Jerusalem in 1099, which preserves the bloodlines of the Merovingian dynasty. This myth was expanded upon and popularized by the 1982 controversial book The Holy Blood and the Holy Grail, and later claimed as factual in the preface of the 2003 conspiracy fiction novel The Da Vinci Code. ... The Priory of Sion myth has been exhaustively debunked by journalists and scholars as one of the great hoaxes of the 20th century. ... Nevertheless, many conspiracy theorists persist in believing that the Priory of Sion is an age-old cabal which acts as a power behind the throne while concealing a subversive secret."
https://en.wikipedia.org/wiki/Priory_of_Sion
