The Diagnostic and Statistical Manual 5th ed. defines obsessions in obsessive-compulsive disorder (OCD) as frequent, persistent, intrusive, unwanted thoughts that provoke anxiety and distress and lead to attempts to neutralize them with either thoughts or actions. However, no systematic review has yet evaluated characteristics that are specific to obsessions occurring in OCD. The aim of the current systematic review and meta-analysis was to investigate the specific features of obsessions occurring in OCD by comparing them to both obsessionally and non-obsessionally-themed intrusions in non-clinical and other clinical populations. Based on a registered protocol, 832 records were found, of which 15 were included in the systematic review and meta-analysis, with a total of 1891 participants. Obsessionally-themed intrusions that occur among those with OCD caused more distress, guilt, negative emotion and interference as compared to similarly-themed intrusions that occur within the general population. The distinction between obsessionally-themed intrusions among those with OCD as compared to those occurring in anxiety and depressive disorder primarily revolves around a higher level of persistence, pervasiveness and distress associated with their occurrence. Further, unacceptability, uncontrollability, ego-dystonicity, alienness, guilt, the form of the intrusion, association with the self and lack of any basis in reality also differentiates between obsessions and intrusions occurring in other disorders. Obsessions share many characteristics with thoughts occurring in other disorders and can be distinguished using a combination of characteristics specific to individual disorders.