My son has been recently diagnosed with OCD. As an example of what happens, he is one of these blokes who shaves his head (a need to feel clean) on a regular basis. A few weeks ago he got sunburnt on his head whilst working in the garden. Although blistered and sore, he still insisted on shaving, breaking open blisters and making a painful mess of his scalp. When his wife tried to talk him out of doing it in the first place, he became quite aggressive (verbally, not physically - he hates violence), so obsessed was he about keeping his head 'clean'.
This compulsion of his, as you can see, is quite irrational, and I think that's where the difference is.