Sprint 0 is a term that some organizations use to describe a phase of work before the first official Sprint, where they do some upfront planning, design, architecture, or analysis1. However, this term is not consistent with the Scrum framework, which does not prescribe any pre-Sprint activities or phases2. Scrum starts with a Product Backlog that contains at least one valuable and feasible Product Goal, and enough Product Backlog items to initiate the first Sprint[3][3]. The Product Owner is accountable for maximizing the value of the product and the work of the Developers, and for managing the Product Backlog4. The Product Owner does not have any specific accountability during Sprint 0, because Sprint 0 is not a part of Scrum.

References:

1: Sprint Zero: A Good Idea or Not?, Scrum Alliance, accessed on December 16, 2023

2: The Scrum Guide, November 2020, p. 5

[3][3]: The Scrum Guide, November 2020, p. 9

4: The Scrum Guide, November 2020, p. 6