Background: Opioid use disorder (OUD) during pregnancy has increased dramatically over the past decade, as have associated adverse maternal health outcomes. Although Medicaid has long been the largest payer for deliveries in the United States, states' decisions to expand Medicaid eligibility to low-income adults has the potential to increase access to care for women in the postpartum period. This study aimed to determine the impact of the 2015 Pennsylvania Medicaid expansion on postpartum insurance coverage and preventive care utilization among pregnant women with opioid use disorder (OUD). Methods: In 2017, we conducted a retrospective cohort study using 2013-2015 administrative Medicaid data provided by the Pennsylvania Department of Human Services. We identified 1562 women with opioid use disorder who had a live birth delivery in a pre-Medicaid expansion or postexpansion study period. We compared length of continuous enrollment in Medicaid following delivery, postpartum visit attendance, and contraception initiation between groups. Results: More women in the post-expansion group remained enrolled in Medicaid at 300 days postpartum, relative to the pre-expansion group (87% vs. 81%). Medicaid expansion was not associated with differences in postpartum visit attendance or contraceptive use. However, women who remained enrolled in Medicaid for at least 300 days post delivery had an increased odds of postpartum visit attendance (odds ratio [OR]: 1.6, 95% confidence interval [CI]: 1.04, 2.4).