The project manager should work with the PMO manager to generate an onboarding plan for the new product owner. This is because the product owner's role is crucial in understanding the product and conveying its requirements to the team. An onboarding plan would help the new product owner to understand the product, the project, and their role better. It would also help in establishing the product owner's credibility with the team.
While options A, B, and D might be part of the subsequent actions, the first step should be to work on an onboarding plan. Assuming the role of the product owner (option A) might create confusion about roles and responsibilities. Scheduling time to share insights (option B) is important, but it should be part of a structured onboarding plan. Organizing a team-building activity (option D) might help in building respect, but it does not address the root cause of the problem, which is the product owner's lack of knowledge.
References: Project Management Body of Knowledge (PMBOK) Guide, 6th Edition, Section 9.1: Plan Resource Management.