The former seminarian, who had known the cardinal since childhood and is the first to speak publicly of his experience, said he had only broken a 30-year silence after recent revelations about the former archbishop's sexual behaviour made it clear his ordeal was not an isolated episode.
The allegation relates to the 1980s when the man was at a seminary college, following four years at Aberdeen's Blairs College for trainee priests. He left the priesthood months after Cardinal O'Brien, then rector of Blairs, allegedly groped and kissed him during a visit.
The former seminarian, who is now in his 50s and has asked to remain anonymous, said he had instigated legal proceedings against the cardinal and the Catholic Church.
He says he is prepared to face "Keith" in court in the hope his actions will encourage others, who may have been reluctant to complain about similar experiences, to come forward.
Cardinal O'Brien was stood down as archbishop of St Andrews and Edinburgh and admitted his sexual conduct had "fallen below the standards expected of me as a priest, archbishop and cardinal" three weeks ago after earlier claims he "behaved inappropriately".
The victim's solicitor Cameron Fyfe said: "I have found this client to be totally credible."
A spokesman for the Catholic Church in Scot-land said: "The Church is disturbed to hear of these allegations. Any complaints raised will be taken very seriously."