This article describes how creating a MOOC (Massive Open Online Course) about ratio and proportion in a collaborative, technological environment during mathematics teacher training, benefited the development of content, pedagogical and technological knowledge for teaching. Twenty-nine trainee mathematics teachers, divided into groups, created teaching units as part of a MOOC in the subject of ratio and proportion. Before MOOC creation, the trainee teachers held misconceptions about the meaning of ratio and proportion, but through a process of reflective thinking, collaboration and peer assessment, they developed a sequence of teaching units about different aspects of the subject. This process allowed them to acquire a broad perspective on the subject, develop their Specialized Content Knowledge (SCK) [Ball, D. L., Thames, M. H., & Phelps, G. (2008). Content knowledge for teaching. What makes it special? "Journal of Teacher Education," 59(5), 389-407] and let go of their own misconceptions. The trainee teachers also integrated into their units realistic situations of ratio and proportion, facilitating involvement and understanding, hence MOOC development also promoted Knowledge of Content and Teaching (KCT) [Ball, D. L., Thames, M. H., & Phelps, G. (2008). Content knowledge for teaching. What makes it special? "Journal of Teacher Education," 59(5), 389-407]. Lastly, the need to select and adapt computerized tools to meet teaching objectives, promoted Technological Pedagogical Content Knowledge (TPCK) [Mishra, P., & Koehler, M. J. (2006). Technological Pedagogical content knowledge: A framework for teacher knowledge. "Teachers College Record," 108(6), 1017-1054]. From their reports, we can gather the trainee teachers learned how to translate theoretical knowledge to practical pedagogical tools.