Single-atom　Fe-N-C　electrocatalysts　have　emerged　as　the　most　promising　oxygen　reduction　reaction　(ORR)　catalyst.　However,　the　low　Fe　loading　and　inaccessibility　of　Fe-N-C　sites　limit　the　overall　ORR　activity.　Here,　we　report　an　efficient　single-atom　electrocatalyst　(Fe-N-C/N-OMC)　with　Fe-N-C　sites　embedded　in　three-dimensional　(3D)　N-doped　ordered　mesoporous　carbon　framework.　Fe-N-C/N-OMC　shows　high　half-wave　potential,　kinetic　current　density,　turnover　frequency　and　mass　activity　towards　ORR　in　alkaline　electrolyte.　Experiments　and　theoretical　calculations　suggest　that　the　ultra-high　ORR　activity　stems　from　the　boosted　intrinsic　activity　of　FeN4　sites　by　graphitic　N　dopants,　high　density　of　accessible　active　site　generated　by　high　Fe　and　N　loadings　and　ordered　mesoporous　carbon　structure　as　well　as　facilitated　mass　and　electron　transport　in　3D　interconnected　pores.　Fe-N-C/N-OMC　also　shows　comparable　ORR　activity　to　PVC　in　acidic　electrolyte.　As　the　cathode　for　zinc-air　battery,　Fe-N-C/N-OMC　exhibits　high　open-circuit　voltage,　high　power　density　and　remarkable　durability.