Stabilizes the manganese cluster which is the primary site of water splitting. Associated with the photosystem II complex. Expression shows a diurnal pattern of oscillation across the 24-hour light-dark, with increased levels during the light period (at protein level) (Ref.3). Down-regulated in the late stationary growth phase as compared to the early stationary and exponential growth phases (PubMed:23291769). Belongs to the PsbO family.