Generally, product owners' role is to tell WHAT TO BUILD instead of HOW TO BUILD it. But there are some situations when product owner must specify some architectural decisions. Let's consider a scenario, years ago; when Sun was looking for the ways to promote Java, they paid companies who made certain applications using Java. But, such decisions should not be taken regularly. Product owner can decide what hardware pieces to be used to make the product economically viable. So product owner can dictate architectural decisions, but they should take good care and should make decisions sparingly, wisely and ideally. To know more, read the article by Mike Cohn (founder of Mountain Goat Software), at: https://www.mountaingoatsoftware.com/blog/can-a-product-owner-dictate-the-architecture