Improving the quality of teaching and learning through high-quality professional development (PD) for schoolteachers has been at the forefront of the educational agenda of many countries around the world, including Oman. This study was designed to examine the perceptions of Omani schoolteachers about the effectiveness of the Ministry of Education's online PD programs in improving their teaching and students' learning as well as determining the elements of effective PD. The study adopted a mixed method design, where 81 schoolteachers who took online courses completed an online survey to gather data regarding their perceptions of the effect of these courses on their teaching practices and students' learning. Four of those teachers were interviewed to collect in-depth data regarding their online learning experience. The findings indicated that the online PD increased their confidence and their ability to face teaching challenges, enhanced their instructional practices and expanded their students' motivation, engagement and participation. In addition, the results of this study underscored that effective online professional development was comprised of seven features: flexibility, active learning, relevance, interaction, adequate and timely feedback, course content, and course design. This study's findings could be used to inform educational leaders and policy makers the importance of designing quality online PD programs that enhance teachers' teaching practices and have an impact on their students' learning. These findings could also be used to inform instructional designers and developers of the elements of effective online PD such as active learning, collaboration, feedback and course content and design.