The Product Owner is accountable for maximizing the value of the product resulting from the work of the Scrum Team. How this is done may vary widely across organizations, Scrum Teams, and individuals.$^1$
The Product Owner is also accountable for effective Product Backlog management (albeit with help from the Scrum Master/Project Manager), which includes:
The Product Owner may do the above work or delegate the responsibility to others. Regardless, the Product Owner remains accountable.
For Product Owners to succeed, the entire organization must respect their decisions. These decisions are visible in the content and ordering of the Product Backlog and through the results of completed stories at the Sprint Review.
The Product Owner is one person, not a committee. The Product Owner may represent the needs of many stakeholders in the Product Backlog. Those wanting to change the Product Backlog can do so by trying to convince the Product Owner.