In this study we aimed to characterize the development of prospective mathematics teachers' noticing in an online teacher education program. Prospective teachers wrote narratives about their own teaching during their traineeship period at schools, and shared them with their colleagues and with the university tutor in an online forum. A narrative is a description of a teaching-learning situation that incorporates reasons about what has happened during the teaching, and justifications of plausible future teaching actions. We analyzed changes in the prospective teachers' narratives by scrutinizing how prospective teachers identify Mathematically Significant Pedagogical Opportunities to Build on Student Thinking (MOSTs) and in which ways they take advantage of them. Additionally, we analyzed whether these changes could be related to the feedback provided in the online forum. The findings indicate changes in how prospective teachers notice their practice, evidenced by the manner in which the MOSTs are taken advantage of, and by the reasons given for their actions. These findings suggest that sharing narratives in online forums and receiving feedback from their colleagues and from the university tutor help prospective teachers to enhance the skill of noticing, leading to changes in their practice. This research provides new knowledge about how online contexts in teacher education programs promote opportunities for collaborative learning, noticing development, and changes in teachers' practices.