A 26-year-old man presents to his GP with significant distress. He describes recurrent, unwanted, intrusive thoughts about germs and contamination. He feels compelled to wash his hands repeatedly throughout the day, often spending over an hour doing so, using harsh soaps which have damaged his skin. These rituals temporarily relieve his anxiety but significantly interfere with his work and social life. He recognises the thoughts and behaviours are excessive. What is the most likely diagnosis?