The Austrian National Opioid Agonist Treatment (OAT-AT) cohort comprises all participants recorded in the pseudonymised statistical sub-registry of the national, open-ended OAT registry since its initiation in January 1, 2002 through December 31, 2019. Data linkage with the Austrian national death registry took place until December 31, 2021. Whereas the OAT registry primarily serves administrative monitoring, OAT-AT links its statistical sub-registry with mortality data to investigate the factors influencing treatment retention and outcomes, as well as mortality rates and causes of death. The cohort includes 26,031 individuals, representing 39,024 OAT episodes (i.e. distinct periods, defined by start and end date during which an individual is authorized by a prescribing physician to receive OAT) with a median episode duration of 914 days and a mean of 1,716 days. The total follow-up time was 278 959 person-years (PY) as of December 31, 2021, with 183 289 PY during OAT (66%) and 95 671 PY (34%) outside of OAT. Median age at cohort entry was 26 years and 76% were male. Findings from OAT-AT will provide new insights into mortality risk and the patient, treatment, and provider factors influencing treatment retention and long-term mortality trends. These findings will facilitate the development of strategies designed to optimize OAT modalities, improve treatment retention, and expand diverse treatment models to enhance access and effectiveness for individuals receiving OAT.