The Opportunity-Propensity Model (OPM) specifies that antecedents (demographics that predict opportunities and propensities occurring before formal schooling), opportunities (exposure to experiences afforded in the home, community, and school), and propensities (pre-existing skills that help children take advantage of opportunities) work in concert to explain individual differences in achievement. Recent work has also pointed towards transactional relations of academic and domain-general skills (working memory), which shares some overlap with the OPM. However, OPM has not been used to explain these transactions. Additionally, researchers have not used the OPM to predict between- versus within-person differences over time, though this might help provide insights on developmental patterns and educational practice. In this study, we use the Early Childhood Longitudinal Study – 2010-2011 to examine the extent to which the OPM accounts for the transaction between mathematics and working memory. • Using random intercepts cross-lagged panel modeling ( Hamaker et al., 2015 ), we found little evidence of cross-lagged relations between math and working memory in Grades 2-5. • Within-person stability (autoregressive terms) were greater in math than working memory. • Stable individual differences in math and working memory were strongly correlated ( r =.70). • We found that socioeconomic status (SES) directly predicts stable, individual differences in math and working memory from Grades 2-5. • SES also indirectly predicts stable individual differences in math (but not working memory) in Grades 2-5 through reading in Grade 1.