In this paper, we put forward the OTTO Model as a means for developing data literacy in pre-service teacher education. The OTTO Model was developed using a combination of quantitative and qualitative data drawn from a three-year study investigating the nature and development of pre-service teachers' data literacy. The aim of the study was to enable pre-service teachers to experience transformative learning during their practice-based professional experiences. Part of this process was to prepare pre-service teachers for their Teaching Performance Assessment (TPA). Requisite skills for TPAs require that pre-service teachers have a high level of assessment and data literacies in order to engage with the TPA tasks. Formative use of the OTTO Model can be embedded into initial teacher education courses allowing more targeted and specific use of data to assist pre-service teachers in developing these requisite literacies.