People with OCD believe that the obsessive thoughts, impulses, or images that they experience are much more than mere thoughts, impulses, or images, they think these things are not merely random occurrences, but horrible warnings of some awful event. That is why people with OCD feel such a need to perform compulsive behaviors; they believe they’re preventing something catastrophic from happening. (Remember the “step on a crack, break your mother’s back” example.)
The other common belief with OCD is that a compulsive behavior has to be performed. People with OCD convince themselves that these behaviors have to be performed either to neutralize an obsession or to have things done the “right” way. Of course, even if they did not do things perfectly, they would be fine, but they have difficulty accepting that or dealing with the thoughts that their inaction would spark about leaving something imperfect.