Developing early number sense and additive reasoning is crucial for academic success. This report examines the impact of the Ongoing Assessment Project (OGAP) on PreK-3 student learning in a large urban district, focusing on the effectiveness of learning trajectory-oriented formative assessment in enhancing mathematics education. OGAP aims to enhance teachers’ capacity to employ research-based student learning trajectories for informed, tailored instruction. The study investigates the impact of one year of OGAP teacher professional development on PreK-3 student math performance, considering the challenges posed by the COVID-19 pandemic, school closures, and the prior year’s remote learning. Impacts of the intervention were assessed via a quasi-experimental design. The study compares the students in the school where teachers received OGAP training to students in a demographically similar matched sample of schools. Teachers who participated in OGAP training reported substantial positive changes in instructional practices. Findings indicate that students in OGAP classrooms outperformed their peers, particularly in basic number knowledge and fact fluency as measured by the STAR curriculum-based measures, but not on the more comprehensive TEMA-3 assessment. These findings offer valuable insights into the relationship between OGAP’s professional development and student outcomes in early math education, especially in the context of a challenging school year. Long-term impacts will be explored in the second and third phase of the implementation.