The Product Owners are accountable to everyone involved in the product. They are accountable to the customers for quality product delivery, they have to MAKE sure the requirements are translated well and there is a mutual understanding of the deliverables. The clients LOOK up to the product owners for their requested WORK and the feedback loops to be taken care of by the product owners. In the same way, the product owners are accountable to the delivery team to make sure they have the right set of requirements to start their work with. 

Along with this, they are accountable for the milestones and the roadmap so that everyone talks the same language. They are even accountable to the management because of it the management who are dealing in terms of finances. The product owner is even accountable to their product backlog, to make it healthy! A lot goes into the accountability of the product owner, it’s like 360-degree accountability that they have performed. Whatever work the DEVELOPMENT team takes up, it is the accountability of the product owner and even making sure the team DEVELOPS the right thing.