Efficient catalysts for the oxygen evolution reaction (OER) are critical to the progress of electrochemical devices for clean energy conversion and storage. Although heterogeneous electrocatalysts have superior activity, it is a great challenge to elucidate electron transfer at surface catalytic sites and intrinsic mechanisms. Herein, we demonstrate a new type of heterostructure electrocatalyst in which Sr 0.9 Ce 0.05 Fe 0.95 Ru 0.05 O 3 fibers are hybridized with in situ grown RuO 2 nanoparticles (SCFR-RuO 2 ). We investigate its unique structure, electron transfer mechanisms related to the highly OER activity by combining experimental and theoretical calculations. Remarkably, SCFR-RuO 2 shows an optimized OER overpotential of 295 mV at 10 mA cm -2 . The promoted electron transfer and OER kinetics are ascribed to the coupling of electronic effects at the SCFR-RuO 2 heterostructure. A strong triangular relationship among overpotential-Tafel slope-work function is proposed to be a potential descriptor of OER activity in SCFR-RuO 2 . These insights provide guidelines for tuning the OER performance via modified work functions in perovskite electrocatalysts.