Electro-catalyzed oxygen reactions, including ORR and OER, play a crucial role in future sustainable energy conversion, enabling a number of electrochemical devices (i.e., fuel cells, metal-air batteries, and water-splitting electrolyzers). The prerequisite for the practical application of these next-generation technologies is appropriate catalyst materials, which can accelerate reaction rate, decrease reaction overpotential, and enhance the selectivity toward the targeted reaction pathway. Nevertheless, the rational design and synthesis of electrocatalysts with favorable intrinsic characteristics are still challenging. Advanced design strategies and mechanism investigations are urgently needed to construct advanced materials for oxygen electrocatalysis. The combination of theoretical and experimental studies working in concert has proven to be a successful strategy in this respect, yielding a framework to understand catalytic trends that can ultimately provide rational guidance toward developing improved catalysts. This PhD thesis targets to rationally design and synthesize high-performance non-precious nano-electrocatalysts for oxygen electrocatalysis via theoretical and experimental means and explore their potential applications in electrochemical energy conversion.[…]