Plaintiff Catherine Tissier says she was forced to work as a servant for the group for virtually no pay.
She says worked 14-hour days, seven days a week, cleaning and serving.
The case brought against Opus Dei in Paris follows a nine-year investigation into the organisation's practices.
The head of a school and the head of a religious retreat north of Paris are the defendants in the case.
Tissier joined the school in 1985, aged only 14, and says she was forced to take her vows and was subject to brainwashing
The defendants deny her accusations and insist she was adequately paid for her work.
Opus Dei has some 85,000 members worldwide and, while it has the backing of the Catholic church, critics believe it is a secretive, manipulative conspiracy.