In a recent randomised double blind pilot study silybum extract demonsrated a similar level of efficacy in the treatment of OCD to fluoextine (an SSRI).
In the study, 52 adults with OCD took silybum (3 x 200mg seed extract / day) or fluoextine (3 x 10mg / day) for 8 weeks. Both were effective in ameliorating symptoms and effected a comparable (strong / significant) decrease in the Yale / Brown score.
Source: Sayyah M, Boostani H, Pakseresht S et al. Comparison of Silybum Marianum (L) Gaerth with Fluoextine in the treatment of Obsessive Compulsive Disorder Prog Neuropsyopharmacol Biol Psychiatry. 2010; 34 (2) 362-5