Prior studies have shown that the variables described in the Opportunity-Propensity (O-P) Framework have successfully accounted for the mathematics and science achievement of students in grades 1-3 and 8-12. The two goals of the present study were to (1) determine whether the O-P Framework could also account for individual differences in the early mathematics skills of low-income, pre-kindergarten children and (2) determine whether latent variables constructed from measured variables would account for performance in the manner specified in the O-P model. The O-P Framework assumes that high achievement in mathematics is a function of three categories of factors: (a) antecedent factors, variables that operate early in a child's life and explain the emergence of opportunities and propensities, (b) opportunity factors, variables that measure a child's opportunity to learn mathematics content at home and school, and (c) propensity factors, variables that capture a child's propensity for learning in terms of self-regulation, motivation, and prior cognitive skills. To test the fit of this model for low-income children during the year before they attend kindergarten, the authors conducted a secondary analysis of achievement and background data from the Early Childhood Longitudinal Study-Birth (ECLS-B) Cohort data set. Structural equation modeling indicated significant associations between the antecedent factor, opportunity factor, and propensity factor, and between the opportunity factor and pre-kindergarten mathematics achievement. The results confirmed the fit of the model and identified the kinds of learning experiences that could promote the acquisition of mathematics skills in low-income children and improve their readiness to learn in first grade and beyond. (author abstract)