This study investigates the transformative potential of a professional development program launched in 2022 to meet the evolving needs of digital teaching competencies within K-12 and higher education sectors. Using the Cultural-Historical Activity Theory (CHAT) and Change Laboratory (CL) framework, the research examines how a MOOC platform designed and iteratively improved a training program for high school teachers, enhancing their online teaching capabilities. Data were collected through meeting minutes, course materials, participant surveys, and platform engagement analyzed using inductive qualitative methods. The study identifies critical contradictions and tensions within the MOOC platform's activity system and explores how these challenges were resolved through collaborative and expansive learning. Findings show that the vibrant learning community established through the MOOC platform was critical in supporting teachers' continuous professional development. Teachers expanded their professional capabilities through peer mentoring, collective problem-solving, and sharing innovative practices, adapting to digital education's changing demands. The study highlights the importance of ongoing support and community-driven learning in sustaining long-term professional growth. The insights gained through CHAT and CL provide fresh perspectives on the dynamics of online teacher training programs, emphasizing the need for continuous improvement to address evolving needs. The study affirms the critical role of vibrant digital teacher communities as hubs for learning, collaboration, and innovation, calling for further CHAT-based research to enhance MOOC platforms and enrich educational exchanges.