Electrochemical water splitting has become a promising method to convert renewable energy into hydrogen fuel, which serves as a storable and clean energy carrier. To reduce the reliance on expensive noble metal catalysts, recent studies have focused on developing cost-effective materials for the oxygen evolution reaction (OER). Among them, multi-metal sulfides have emerged as attractive candidates due to their rich active sites, decent electrical conductivity, and relatively weaker metal–sulfur bonds. Compared to metal oxides, metal sulfide materials often show better catalytic activity and thermal stability, making them suitable for OER applications. In this work, a colloidal one-step heat-up synthesis method was used to produce monodisperse metal sulfide nanocrystals with tunable composition and morphology. In the first part, we developed a synthetic strategy to achieve star-shaped anisotropic growth of spinel-phase high entropy NiCoFeMnInS nanocrystals, with an average core size of 22 nm and arm length of 9 nm. These shape-controlled high-entropy nanocrystals provided increased surface area and exhibited a low OER overpotential of 313 mV, maintaining stable performance at 10 mA/cm² for 14.5 hours, showing promising catalytic activity. In the second part, we synthesized ternary medium entropy metal sulfide nanocrystals NiCoFeS with an average size of 8.01 nm (±12%), along with the in-situ formation of a In2S3-based FeMnInS nanosheet support. This heterostructured material exhibits enhanced OER catalytic activity of nanocrystals, and shows a competitively low overpotential of 285 mV and maintained stable operation over 23 hours at 10 mA/cm².