A strong relationship between article background and educational outcomes fuels a negative inequality cycle. This paper explores the interplay between student socioeconomic status and educational outcomes, and the mediating role of Opportunity-to-Learn (OTL) in high- and middle-income countries. Using data from PISA 2012, we find that the relationship between OTL and mathematics achievement is mostly positive. The magnitude of this relationship is higher in more affluent societies. Our results raise the possibility that unobserved teacher quality, school leadership, or other features of schools in lower-income countries make OTL less productive than in more developed nations. This lends support to the "complementary process" hypothesis and the notion that success (or riches) begets success. The analysis also shows that OTL mediates the effect of individual socioeconomic and cultural background on educational outcomes. This finding lends support for a policy emphasis on classroom-level practices as one key lever for improving learning and upward mobility for disadvantaged students.