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Abstract High-entropy oxides (HEOs) offer vast compositional design space for discovering emergent functionalities, yet their controlled nanoscale synthesis remains challenging. Here, we develop a generalizable colloidal strategy that enables precision synthesis of HEO nanocrystals with compositions spanning quinary to septenary systems. Mechanistic studies reveal that differences in precursor reactivity drive a multistage growth pathway and that cooperative multimetal chemistry─where one metal initiates single-phase nucleation and a vacancy-forming metal promotes cation redistribution during growth─enables homogeneous multication incorporation. The resulting rocksalt HEO nanocrystals can be transformed into spinel phases and exhibit excellent oxygen-evolution reaction activity. Machine-learning-accelerated theoretical analysis and operando characterization identify Co–Co bridge sites on spinel {111} facets as the dominant oxygen-evolution active motifs operating through a lattice oxygen-mediated mechanism. These findings establish guiding principles for controlling nucleation, cation mixing, and active-site formation in compositionally complex oxides, enabling the rational design and data-driven optimization of high-entropy materials.