Obsessive compulsive disorder (OCD) is a mental illness that has penetrated public consciousness. However, the extent to which OCD and obsessive compulsive personality disorder (OCPD) can constitute debilitating conditions that adversely affect most aspects of a person's functioning and quality of life are not so well known, including as to how they can impair the capacity to give reasoned consideration to conduct options and the consequences of choices. Little scholarship exists about the legal repercussions of OCD and OCPD and, in particular, their potential relevance for both assessments of criminal responsibility and criminal culpability. This article commences to redress that deficit, outlining contemporary clinical knowledge about the disorders that is relevant to the legal context and identifying important judgments by courts in the United Kingdom, Ireland, Canada, Australia, New Zealand and India which have dealt with the potential impact of OCD and OCPD, in particular for decisions at the sentencing phase of criminal proceedings. It calls for better awareness of OCD and OCPD on the part of forensic mental health practitioners, criminal law practitioners and members of the judiciary.