In this article we examine obsessive compulsive disorder (OCD). We examine and reject two existing models of this disorder: the Dysfunctional Belief Model and the Inference-Based Approach. Instead, we propose that the main distinctive characteristic of OCD is a hyperactive sub-personal signal of being in error, experienced by the individual as uncertainty about his or her intentional actions (including mental actions). This signalling interacts with the anxiety sensitivities of the individual to trigger conscious checking processes, including speculations about possible harms. We examine the implications of this model for the individual's capacity to control his or her thoughts.