Paliperidone has a number of advantages over SRIs (the commonly prescribed medication), including fewer drug interactions and better tolerability. This study is designed to determine whether adding paliperidone to an existing medication (an SRI) is more effective in reducing OCD symptoms compared to taking a placebo and an existing medication (SRI). Patients accepted into this study will receive physicals, psychiatric exams, labs, ECG, and study medication all at no cost to the patient. There will be 8 visits over 10 weeks. For each completed study visit, $15 will be reimbursed to the patient for travel and meal expenses.