The project management office is not clearly defined and organizations need to come up with a definition of responsibilities that suits them.
In general, however, a PMO is either a physical organizational unit or a virtual one (aided by software) that is responsible for the overall project management controlling and the creation, implementation and development of a company's project management system. It can also provide project assistance and common responsibilities are e. g.:
One goal of the PMO therefore can be to establish and provide guidelines, templates, and general resources that enable all project stakeholders to fulfill their project roles and work effectively.
The people working in the PMO need project management expertise and knowledge of project management methods and often hold project management certifications. They also need expertise in digital document management or project management aides such as project management software, e. g. for the creation of project schedule templates.
The PMO is usually also closely aligned to the overall business strategy of the organization and can often be tasked with providing reports on the project portfolio of the organization. It may also be tasked with implementing changes to how projects are planned, monitored and executed and assist project managers with the change management.
Read up on further details on the role and tasks of a PMO.