The importance of the opportunity to learn (OTL) for mathematics achievement has been extensively researched. However, there were still unanswered questions regarding OTL's measurement, analytical level, and relationship with motivational beliefs. To fill in the gaps, we aimed to (1) scrutinize the reliability and validity of OTL, (2) investigate the multilevel nature of OTL, and (3) explore the relationship among OTL, self-efficacy, and mathematics achievement. We used Programme for International Student Assessment (PISA) 2012 data provided by 470,745 15-year-old students from 63 economies. Psychometric analyses and doubly latent multilevel structural equating modeling were conducted. Results indicated that (1) OTL measured by familiarity with mathematical concepts displayed high reliability and validity, (2) student-reported OTL exhibited high reliability at the school-level, (3) OTL significantly predicted achievement both at student- and school-level, and (4) OTL mediated the relationship between self-efficacy and mathematics achievement at student-level. Theoretical and practical implications are discussed.