The BABOK (Business Analysis Body of Knowledge) guide offers an extension for agile, and in an article at Project Times, Liza Laya discusses how this extension might be applied to PMOs. She considers the PMO and agile according to strategy, initiative, and delivery horizons respectively. On the strategic horizon, the agile PMO reacts to the environment and customer expectations to make fast changes. On the initiative horizon, the agile PMO identifies needs that must be satisfied, as well as how to satisfy them. And on the delivery horizon, the agile PMO builds a prioritized backlog informed by business goals, which it continuously updates over time.
For the full picture (literally—there is a diagram), you can view the original article here: https://www.projecttimes.com/articles/extending-the-agile-extension-to-pmos.html