Abstract: The recent International Classifications of Diseases-11 (ICD-11) distinction of complex- post-traumatic stress disorder (C-PTSD) from post-traumatic stress disorder (PTSD), has highlighted a research gap in exploring how C-PTSD may relate to obsessive-compulsive disorder (OCD) differently than PTSD. Mental health disorders and comorbidities appear to be greater in military veterans compared to the general population. Thus, this study aimed to explore potential differences in OCD severity between probable PTSD and probable C-PTSD in a national clinical sample of UK military veterans. Data from 428 veterans were analysed using a previously collected dataset. The survey assessed sociodemographic characteristics, military experiences, physical and mental health, and well-being. Results indicated significant differences in OCD severity between probable PTSD and probable C-PTSD. OCD severity significantly increased as C-PTSD symptom severity increased for veterans with probable C-PTSD. Though no significant association was identified between OCD severity and PTSD scores within the probable PTSD group, this finding should be interpreted with caution, as the small sample may have limited statistical power. Greater C-PTSD severity significantly predicted greater OCD severity, but PTSD scores did not. Disturbances of self-organisations (DSO) symptoms within C-PTSD were more strongly associated to OCD severity compared to PTSD symptoms, indicating a seemingly complex interplay between C-PTSD's cluster of symptoms and OCD severity. Future research should focus on replication involving larger veteran samples and the general population, incorporating clinician-administered assessments alongside self-report measures to enhance diagnostic accuracy.