This study examined the impact of Open Educational Resources (OER) based professional development of secondary school biology teachers in Samtse Dzongkha in Bhutan. It highlights the importance of teaching and learning biology in secondary classrooms, emphasising its relevance to students' understanding of science, the environment, and their future academic and career choices. Recognising the challenges faced by teachers and students in biology education, STEM teacher educators from Samtse College of Education underwent professional development using OERs and online sessions facilitated by experts from Tata Institute of Social Sciences (TISS) in India. These educators collaborated with peers from Tanzania, Nigeria, and TISS to develop six-week-long OERs on specific biology topics, which were then shared with selected in-service and pre-service biology teachers in Bhutan through Moodle platform. Additionally, an online Community of Practice (CoP) was established on Telegram to support the ongoing professional development. The study employed a mixed-methods approach, utilising pre and post-test scores, lesson plans, reflections, and interviews to assess the impact of the OERs on teachers. The CoP network was also analysed to understand communication patterns and knowledge sharing within the community. The results from pre-test and post-test assessments across these modules showcased marginal improvements in teachers' subject content knowledge. Detailed analysis of lesson plans and reflections revealed evolving proficiency in various pedagogical domains. Notably, many teachers sought technological integration, demonstrating an eagerness to utilize digital tools for enhancing student engagement. Despite these positive trends, a significant gap in high expertise levels among teachers was observed, particularly in the Genetics module. This hints at an underlying need for enriched professional development opportunities in specific content areas. Therefore, findings of this study concludes by recommending educational institutions to support continuous professional development for biology teachers through the integration of OERs, thereby improving the quality of biology education.