This study explored the experiences of 13 core teachers of Enhancing Teacher Education Program when supporting their peers to fulfill online learning courses via the learning management system. It used a qualitative and interpretive methodological approach through semi-structured interviews to investigate the forms and factors affecting peer support in online learning. Research findings showed two forms of peer support via online and offline modalities. Exploring enabling factors for peer support in online learning is a great support of local administrative agencies and school leaders, core teachers’ personal experiences and reputations. However, core teachers identified a number of challenges including the limited digital skills and low learning motivation of recipients, core teachers’ heavy workload, and unreasonable compensation. These research findings contribute to the knowledge of teacher professional development and put forward recommendations to improve the online training courses and the work of core teachers in Vietnam.