The O-RAN Alliance is a worldwide effort to reach new levels of openness in next-generation virtualized radio access networks (vRANs). Initially launched by 5 major mobile carriers a couple of years ago, it is nowadays supported by over 160 companies (including 24 mobile operators across 4 continents) representing an outstanding example of how operators and suppliers around the world can constructively collaborate to define novel technical standards. In this paper, we provide a summary of the O-RAN Alliance RAN architecture along with its main building blocks. Then, a practical use case exploiting the AI/ML-based innovations enabled by O-RAN is presented, showcasing its disrupting potential. Based on this, the defined interfaces and services are described. Finally, a discussion on the pros and cons of O-RAN is provided along with the conclusions.