High-entropy perovskite oxides (HEPOs) have recently emerged as a promising class of oxygen electrocatalysts due to their configurationally stabilized crystal structures, tunable electronic properties, and compositional flexibility. However, the role of individual B-site cations and their synergistic interactions remains insufficiently understood, particularly regarding bifunctional activity in oxygen evolution (OER) and oxygen reduction (ORR) reactions [1-3]. In this study, we systematically investigate a series of orthorhombic La(FexCoyMnzCr0.2Zn0.2)O3−δ HEPOs (x, y, z = 0.3, 0.2, 0.1 alternately) synthesized via a rapid Joule-heating method to unravel the intrinsic link between cation ratios, lattice distortions, oxygen vacancy concentrations, and bifunctional electrocatalysis. All HEPO compositions maintain the same single-phase orthorhombic structure and similar particle sizes, allowing a direct evaluation of how B-site cation redistribution influences intrinsic properties. Structural refinement and XPS show that adjusting Fe/Co/Mn ratios tunes the average B-site ionic radius and local electronic environment, generating controlled lattice distortions and oxygen vacancies without altering transition-metal oxidation states. Among all materials, the Co-rich/Mn-poor La5M-Co/Mn composition exhibits the highest oxygen-vacancy concentration and optimal metal–oxygen coordination, leading to superior OER activity with a 296 mV overpotential at 10 mA cm⁻², low charge-transfer resistance, enlarged electrochemical surface area, and a predominantly four-electron ORR pathway, yielding the lowest bifunctional index (1.042 V). When implemented in zinc–air batteries, La5M-Co/Mn delivers a smaller charge–discharge gap, higher peak power density (82 mW cm⁻²), greater capacity (390.7 mAh at 5 mA cm⁻²), and enhanced rate capability and cycling stability, demonstrating that cation-ratio engineering effectively boosts lattice distortion, oxygen-vacancy formation, and overall electrochemical performance. Overall, this work demonstrates that precise manipulation of B-site cation ratios without altering the underlying perovskite framework is a powerful strategy to engineer oxygen vacancies, optimize metal–oxygen coordination, and significantly boost bifunctional electrocatalytic performance. The insights presented here establish a clear structure–composition–property relationship for HEPOs and underscore the potential of cation-ratio engineering as a scalable, fundamental design principle for next-generation energy-conversion devices, especially high-performance rechargeable zinc–air batteries.