A 12-year-old boy is brought to your clinic for being very strange in his behaviours. He is late to school because he has to be in the shower for more than 2 hours. He feels compelled to use the soap repeatedly to clean off the 'germs' which he is worried off. He feels compelled to do this although he thinks that it is senseless. He checks his bag for his pen every 5 minutes. He has always struggled to complete his exams in time. Which of the following is the MOST likely diagnosis?