Designing earth-abundant and durable catalysts for the oxygen evolution reaction (OER) is vital for sustainable energy conversion and carbon-neutral technologies. Herein, we present a modular design strategy to construct an efficient sulfide/oxide heterostructured electrocatalyst through a facile sulfidation treatment of natural ilmenite. The resulting catalyst integrates high intrinsic activity, good electrical conductivity, and long-term operational stability. Benefiting from the synergistic coupling between FeS₂ and TiO₂, it requires only an overpotential of 230 mV to achieve a current density of 10 mA cm^-2 and retains its performance over extended electrolysis in alkaline media, showing nearly 20-fold higher OER activity than pristine ilmenite at 300 mV. Comprehensive structural and electrochemical analyses reveal that the superior performance originates from an accelerated surface self-reconstruction process, where lattice-sulfur leaching promotes the in situ generation of FeOOH species that serve as the genuine active phase. Meanwhile, the residual FeS₂ modulates surface electronic structures, facilitating charge transfer and enhancing conductivity, while the TiO₂ scaffold maintains mechanical integrity and prevents corrosion. This synergistic interplay ensures efficient charge separation, fast reaction kinetics, and excellent durability. This work establishes a scalable and eco-friendly route to convert natural minerals into high-efficiency heterostructured catalysts, providing a sustainable materials platform for renewable-energy conversion and storage applications.