Excellent question! Clinically, I find that there is a problem with doing gradual exposure with OCD. With exposure to fears and phobias, both gradual exposure and flooding tend to be effective and the choice can be made based on logistical considerations (timeframe, preference of patient, etc). However, with OCD, often if you try to use gradual exposure, the patient can come up with ways to make the reduced rituals more effective at reducing their anxiety, thus defeating the purpose of the exposure. For example, if you try to gradually reduce the number of handwashings a handwasher engages in per day, they could find ways to make the washings they do perform feel more effective and, in effect, develop new and better rituals. Thus, my preference, when feasible, is to go with flooding with OCD. That stated, one still needs to consider the willingness and ability of the patient to followthrough with the treatment procedures, so that the ideal is often not an option. In those cases, one might end up doing more of a modified flooding where you start with as high up on the hierarchy as they patient's situation will allow and move up as rapidly as possible. I would also love to hear other professional's responses to this question!