Developing　high-performance　and　low-cost　oxygen　evolution　reaction　(OER)　electrocatalysts　is　critical　for　implementing　industrial　H2　production.　Herein,　ultrathin　Fe-Mn-O　hetero-nanosheets　consisting　of　crystalline　e-Fe2O3　intercalated　in　amorphous　Fe-doped　6-MnO2　have　been　fabricated　through　facile　alkali　hydrolysis　of　FeMn-based　coordination　polymer.　Abundant　Mn3+　active　sites　and　oxygen　vacancies　are　generated　in　Fe-doped　6-MnO2　layer　due　to　in-layer/interlayer　reformations,　where　interstitial　e-Fe2O3　is　engineered　to　reinforce　stability　and　conductivity.　The　optimal　0.5Fe-Mn-O　nanosheets　exhibit　superb　activity　with　low　overpotentials　of　258　(322)　mV　at　10　(300)　mA　cm-2　for　OER　together　with　strong　stability　under　large　current　density　over　100　h,　ranking　it　among　the　best　MnO2-based　electrocatalysts.　Importantly,　in　situ　interlayer　reconstruction　from　e-Fe2O3　to　fl-FeOOH　during　OER　has　been　unveiled,　the　evolved　fl-FeOOH　triggers　strong　interfacial　electronic　coupling,　which　synergizes　with　oxygen　vacancies　and　Fe3+　dopants　to　modulate　surface　electronic　structure　of　6-MnO2　for　intermediates　adsorption.