Despite the growing success of open-source software ecosystems (SECOs), their sustainability depends on the recruitment and involvement of ever-larger contributors. As such, onboarding, i.e., the socio-technical adaptation of new contributors to a SECO, forms a significant aspect of a SECO’s growth that requires substantial resources. Unfortunately, despite theoretical models and initial user studies to examine the potential benefits of onboarding, little is known about the process of SECO onboarding, nor about the socio-technical benefits and drawbacks of contributors’ onboarding experience in a SECO. To address these, we first carry out an observational study of 72 new contributors during an OpenStack onboarding event to provide a catalog of teaching content, teaching strategies, onboarding challenges, and expected benefits. Next, we empirically validate the extent to which diversity, productivity, and quality benefits are achieved by mining code changes, reviews, and contributors’ issues with(out) OpenStack onboarding experience. Among other findings, our study shows a significant correlation with increasing gender diversity (65% for both females and non-binary contributors) and patch acceptance rates (13.5%). Onboarding also has a significant negative correlation with the time until a contributor’s first commit and bug-proneness of contributions.