PMO is defined around three pillars.
1/ The PMO brings support to the management of the projects. It guarantees the quality of the execution of each project.
2/ The PMO manages the program portfolio. It provides a consolidated view on the portfolio of projects, enables prioritization and arbitration between projects and check for consistency and dependencies in the program.
3/ The PMO defines the governance and tools of project management. It defines the best practices in terms of roles and responsibilities, processes, tools and communication and it trains project managers.