The rise in online professional development (PD) opportunities for teachers, accelerated by the COVID-19 pandemic, necessitates an examination of the effectiveness of these online PD programs and the design elements that best enhance teachers' knowledge. This review synthesizes findings from nine studies that systematically investigate experimental and observational research evaluating formal online PD programs for teachers. The studies employed various research designs, including randomized controlled trials (RCTs), pretest-posttest designs, and mixed-method approaches. Key characteristics of the studies were analyzed, including publication dates, originating nations, targeted demographics, research objectives, program information, study designs, participants, data collection methods, and reported results. The findings indicate that effective online PD programs enhance teachers' content knowledge (CK), pedagogical content knowledge (PCK), instructional practices, self-efficacy, and professional competence. Despite the benefits, the review also identifies several barriers to the effective implementation of online PD. These include organizational challenges, time constraints, technical difficulties, and issues related to sustaining new practices. The need for continuous support, practical application of new knowledge, and interactive elements to foster engagement and collaboration among educators is also highlighted. The review concludes with recommendations for future online PD programs, emphasizing the importance of well-designed, flexible, and supportive structures that address the unique needs of educators. The profound influence of COVID-19 on the adoption of online PD underscores the necessity for further research into effective design components and support mechanisms. This review serves as a valuable resource for program designers and decision-makers aiming to enhance the effectiveness of online PD for educators. [This paper was published in: "EJER Congress 2024 International Eurasian Educational Research Congress Conference Proceedings," edited by Senel Poyrazli, Ani Publishing, 2024, pp. 61-77.]