Product Owner & the Team
What is the biggest problem that most teams face? That, at the high level (value) or the low level (details), they don't understand what the customer wants well enough.
And who is mainly responsible for managing the flow of this 'business information' into the Team? The PO of course.
And this is a never-ending job. For example, at the lowest level imaginable, as they complete each step of work, he should be giving feedback: "well, that was not quite what I meant", meaning really "I don't think it is quite what the customer will want."
The feeding of the team, this feedback to the team of Devs (meaning, in overly simple terms, creators and testers) is essential. And must be done daily.
When we start with a past tradition of thinking and working, there are many obstacles to this 'union' or yoking of the PO with the Devs. One is that the Devs wonder "what is that guy doing hanging around here?" Another is that the PO feels kind of weird around all these geeks and their geek-talk.
But both sides need and will eventually learn how to live and work with each other. It takes time.
Only together, as a full team, can they win.