This thesis utilizes anonymized patient health records from one of the largest mental health service providers in Europe and explores mortality risk factors, at both patient-level and service-level, in individuals with opioid use disorder (OUD) enrolled in secondary drug and alcohol treatment. The thesis explores associations between psychological wellbeing, comorbid diagnosis of personality disorder (PD), serious mental illness (SMI) and alcohol use disorder (AUD), in relation to mortality in opioid dependence. Specific risk situations in time and context are also explored, with investigations of clustering of deaths in the period immediately after transfer of patients and their care and after end of opioid substitution treatment (OST) in a cohort of opioid dependent individuals in specialist addiction treatment. The thesis also investigates if routine brief risk assessments given to OUD patients can predict all-cause or cause specific mortality and determine if these risks may be modified by admission to services.