Although shadowed by the institutionalization of contemporary schooling, out-of-school time and space encompass a variety of learning possibilities and are socially organized to enable and maintain educational privileges for some students but not for others. Out-of-school time (OST) curricular instructions have been expanding globally and more visible in East Asia. Relying on multilevel ordered logistic regression and generalized propensity score analysis; this study investigated the participation patterns and effectiveness of OST mathematics education in Shanghai, China. Family socioeconomic background and opportunity-to-learn indices of mainstream schooling were found to strongly link to participation in OST mathematics education. Higher socioeconomic status and more household resources were associated with higher likelihood of participating in OST mathematics education. Negative school-level opportunity-to-learn indices, such as problematic classroom disciplinary climate and insufficient mathematics instructional time, correlated with greater odds of OST mathematics participation. In addition, this study found a significant relationship between OST mathematics learning and mathematics study behaviors, with higher intensity of OST mathematics instruction triggering more positive study ethics and habits. Yet, no significant association between OST mathematics education and PISA mathematics achievement was identified. These findings profoundly illuminated the multi-layered interconnections between formal schooling and OST education and contributed to the discussions about structures of educational equality during out-of-school time.