Yes, if there is a SINGLE PO, can also take care of the RPO role, which is Release Product Owner. As there is only one person taking up the responsibility, the product owner will perform the duties towards the scrum team and also towards a higher role, let us see what an RPO does and what are the essential responsibilities:

• Creating a clear STATEMENT of vision, direction, release PURPOSE and goals
• Managing overall Product Backlog and publishing the Product Backlog so that the TEAMS can pull the work items in their release commitment.
• The features should be mapped with product roadmap to make sure the end result builds up as expected.
• Once the initial set of requirements are up, talk to the stakeholder and get their buy-in on Product Backlog
• Prioritization of Product Backlog and keeping it healthy throughout.
• Prepare appropriate Product Backlog to drive release planning
• Working on getting the ongoing release plan forecasting maybe for six months or a years’ time (as per the organization and client delivery expectations)
• Deployment & release readiness checklist
• Market launch split out to Product Management