Mn-Based oxides, particularly CaMn oxides, have recently attracted significant practical interest as a new class of catalyst due to their elemental and structural similarity to the natural oxygen evolving cluster (OEC) in photosynthetic plant cells. However, their performance as oxygen-generating anodes in photoelectrochemical cells has not been studied in detail. In this work, ultra-fine particles of amorphous MnO2, crystalline MnO2 nanorods, Ca2Mn3O8, CaMn2O4 and CaMnO3 were synthesised using a green and scalable mechanochemical method. The particles were comparatively studied as water oxidation photocatalysts in a photo-electrochemical cell at near-neutral pH. The oxides were immobilized on the anode surface using an organic, conducting polymer that facilitated electron exchange and catalytic turnover in a manner similar to redox-active tyrosine in the OEC. The differences in their photocatalytic performances were evaluated in terms of: (1) structural similarities to the natural OEC, (2) Mn oxidation state, (3) crystal structure, (4) specific surface area, (5) electron energy state, and (6) the presence/absence of Ca. The results confirmed the importance of having a local structure that is as similar as possible to the natural OEC cluster, including the presence of Ca. However, it also indicated that it is simplistic to focus only on this feature. The other factors listed above may also play a critical role in performance. Future design of biomimetic catalysts for solar fuel production needs to consider and concurrently optimize all of the relevant influences.