Abstract: Importance: Hospitalization is a critical period for patients with opioid use disorder (OUD), marked by increased risk of mortality after discharge. Objective: To determine the timing and magnitude of opioid-related and all-cause mortality risk following hospital discharge among veterans with OUD. Design, Setting, and Participants: Retrospective cohort study of 98,890 veterans with OUD hospitalized in the Veterans Health Administration, with mortality tracked for 1 year post-discharge. Main Outcomes and Measures: Interval-specific risks of opioid-related and all-cause mortality were estimated using adjusted survival models for periods up to 90 days post-discharge, compared to 91–365 days. Results: Within 1 year, 7.8% of patients died, including 848 opioid-related deaths. The first 7 days post-discharge carried nearly triple the risk of opioid-related death (HRR, 2.85; 95% CI, 2.5–3.3) and all-cause death (HRR, 2.80; 95% CI, 2.5–3.2) compared to later periods. Few patients received naloxone (5.2%) or buprenorphine (6.5%) around hospitalization. Conclusions and Relevance: Mortality risk is highest in the week after hospital discharge for veterans with OUD. Enhanced transitional care and timely addiction treatment are urgently needed to reduce post-discharge deaths.