Abstract Although previous research has demonstrated the effectiveness of initiating medications for opioid use disorder (MOUDs) during incarceration, there has been very little adoption nationwide of this practice. Extended-release buprenorphine (XR-B) is a promising intervention for the treatment of OUDs in carceral settings. Individuals with OUDs re-entering the community are at an elevated risk for overdose and death from relapse to opioid use among other opioid-related harms. This proposal builds on the current investigators experience conducting clinical trials using both XR-B and sublingual-buprenorphine-naloxone (SL-B) with individuals in jail and prison. This proposed study is a randomized controlled trial of XR-B vs. SL-B in a large metropolitan jail. An open-label design will randomly assign 240 adults with moderate-to-severe OUDs who are soon-to-be-released from jail to either XR-B (n = 120) or SL-B (n = 120) treatment in jail followed by 6-months of post-release buprenorphine treatment, a 7-month safety visit, and final long-term follow-up at 12-months. The study has three aims: Aim 1. Compare the effectiveness of XR-B vs. SL-B in terms of: Primary Outcome. (a) illicit opioid use (i. urine toxicology; ii. self-reported days of opioid use using Timeline Followback; and iii. time to opioid relapse). Secondary Outcomes. (b) retention in buprenorphine treatment (i. days receiving buprenorphine and ii. time to treatment dropout); (c) other illicit substance use (i. urine toxicology; ii. self-reported days of illicit substance use using Timeline Followback; (d) overdose events (non-fatal and fatal); (e) quality of life (i. physical health; ii. mental health); (f) HIV risk behaviors (i. sexual risk behavior; ii. needle use or sharing); and (g) criminal activity (i. crime days; ii. re-arrest; iii. technical violations; iv. re-incarceration). Aim 2. To calculate the cost to the state and/or jail/city health system of implementing XR-B and SL-B, and determine the relative value, including the costs associated with the interventions in the community, from a county and state-policymaker and societal perspective. Aim 3. Explore barriers and facilitators to XR-B/SL-B implementation in jail: (1) dose induction; (2) diversion and procedures for reducing diversion; (3) continuity of care after release or transfer to another facility; (4) staffing (both custody and medical) needs for daily versus XR-B buprenorphine dosing; and (5) patient preference for XR-B versus SL-B. The proposed study would be innovative because it would be the first large scale RCT in the US assessing effectiveness and cost effectiveness of XR-B versus SL-B with individuals in a jail who are re-entering community. Understanding how to expand acceptance of buprenorphine in jails, has far-reaching implications for expanding treatment access in jail.