It can be crippling. Some people who get up just to go to the bathroom and wash their hands, go through a 3-4 hour ordeal. The number of hand washings some people do per day can leave their skin brittle and in need of treatment. Some people can't function anywhere because of their compulsive need to ritualize their behavior. Obsessive thinking can be unpleasant to extremely painful.
Some people can be obsessive and ruminate about anything from numbers to cleaning.
They can function as long as it does not interrupt their life and they can get through the day. For other people, obsessive thoughts can be terrifying and overwhelming. Many times the thoughts are not of pleasant issues but are foreign to the person- such as a wish to do some type of physical harm or have some type of unpleasant sexual thoughts, which keep running though their mind. It can get so tormenting at times that it can lead to suicidal wishes which then itself can become an obsession. For years the only approach to treat OCD was through behavior modification.