The Open Radio Access Network (Open RAN) aims to enable disaggregated, virtualized, programmable, and data-driven intelligent network with open interfaces to support various real-time and non-real-time applications for different classes of users and multiple industry verticals in beyond 5G and 6G networks while providing interoperability among multi-vendor network functions and components. In this article, we first discuss the evolution of RAN and then the O-RAN Alliance standardization activities and objectives to provide a comprehensive overview of O-RAN from a standardization point of view. Then, we discuss the O-RAN security aspects, use cases, deployment aspects, and open source projects and related activities in other forums. Finally, we summarize the open issues, challenges, and future research directions to explore further for in-depth study and analysis.