This　study　presents　a　systematic　theoretical　and　experimental　investigation　aimed　at　optimizing　the　photocatalytic　oxygen　evolution　reaction　(OER)　performance　of　poly(heptazine　imides)　containing　metal　cations　(PHI-Ms).　Deploying　advanced　density　functional　theory　(DFT)　and　time-dependent　DFT　calculations,　time-resolved　spectroscopy,　and　photocatalytic　OER　measurements,　we　established　key　design　principles　for　enhancing　the　OER　activity:　(i)　favoring　Co　and　Fe　cations　in　the　+III　oxidation　state　to　improve　light-harvesting　and　reduce　the　Gibbs　free　changes;　(ii)　minimizing　the　content　of　K+co-cations,　which　negatively　affect　catalytic　performance　by　increasing　the　desorption　Gibbs　free　energy　of　O2and　promoting　charge　localization;　(iii)　avoiding　high　concentrations　of　metal　cation　to　limit　agglomeration　and　mitigate　charge　recombination;　and　(iv)　exploring　oxygen　doping　of　the　PHI　layer　by　substituting　N　atoms　located　away　from　the　metal　active　sites,　thereby　enhancing　charge　separation　and　promoting　hole　generation.　These　insights　offer　valuable　design　guidelines　for　the　development　of　PHI-M　systems　with　improved　photocatalytic　performance,　advancing　the　potential　of　earth-abundant　catalysts　for　practical　water-splitting　applications.　©　2025　The　Authors.　Published　by　American　Chemical　Society