Despite expansions in the number of insured individuals and the increased availability of treatment for opioid use disorder (OUD), over 70% of adults with OUD do not receive care. Barriers to accessing OUD care exist at the individual and structural level. Considerable gaps persist in our understanding of these access barriers as they can vary by sociodemographic and geographic factors. Further, OUD frequently co-occurs with depression. Not only are individuals with OUD and co-occurring depression (“dually diagnosed”) harder to treat, but if left untreated, they are at greater risk of adverse health outcomes compared to those with OUD or depression alone. Treatment pathways for both OUD and depression in those dually diagnosed are particularly complicated and understudied. There is a critical need to develop a holistic 360-degree view of OUD care in this population. Such a view is necessary to provide context for understanding determinants of access to care and treatment utilization and to develop nuanced interventions for improving equity in care. Thus, I aim to accomplish two distinct goals connected via the principle of providing a whole- person view while advancing health equity. The specific aims are to: develop and validate an OUD- specific Resource Index (OUDRI) (Aim 1); examine, characterize and compare distinct trajectories of antidepressant treatment and medications for OUD (MOUD) combinations among publicly-insured individuals with OUD and depression (Aim 2); and evaluate the association between the treatment trajectories identified in Aim 2 and OUD-related outcomes and depression-related outcomes (Aim 3). The research strategy for Aim 1 is to use publicly-available datasets to generate a composite index that quantifies the potential to access resources for OUD treatment. The research strategy for Aims 2 and 3 is to use a national sample of adult Medicaid beneficiaries with OUD and depression from 2010 through 2018. I will link the OUDRI developed in the first Aim to the claims dataset to contextualize each treatment utilization trajectory and associated outcomes. The expected study impact is that it will improve our understanding of the totality of care and health outcomes for people with OUD and those dually diagnosed with depression. This research addresses the Agency for Healthcare Research and Quality (AHRQ)'s priority populations by targeting Medicaid enrollees with OUD, which includes low-income and racial/ethnic minority populations needing chronic care. This dissertation addresses AHRQ's research priority areas to harness data to improve the quality of OUD care and outcomes by identifying optimal longitudinal treatment pathways in the dually diagnosed population. Consequently, with its fundamental focus on achieving equity in OUD care delivery, this project is responsive to SEN NOT-HS-23-013.