Simultaneous study of magnetic and electrocatalytic properties of cobaltites under extreme conditions expands the understanding of physical and chemical processes proceeding in them with the possibility of their further practical application. Therefore, La_0.6Sr_0.4CoO₃ (LSCO) nanopowders were synthesized at different annealing temperatures t_ann = 850-900 °C, and their multifunctional properties were studied comprehensively. As t_ann increases, the rhombohedral perovskite structure of the LSCO becomes more single-phase, whereas the average particle size and dispersion grow. Co³⁺ and Co⁴⁺ are the major components. It has been found that LSCO-900 shows two main Curie temperatures, T_C1 and T_C2, associated with a particle size distribution. As pressure P increases, average ⟨T_C1⟩ and ⟨T_C2⟩ increase from 253 and 175 K under ambient pressure to 268 and 180 K under P = 0.8 GPa, respectively. The increment of ⟨dT_C/dP⟩ for the smaller and bigger particles is sufficiently high and equals 10 and 13 K/GPa, respectively. The magnetocaloric effect in the LSCO-900 nanopowder demonstrates an extremely wide peak δT_fwhm > 50 K that can be used as one of the composite components, expanding its working temperature window. Moreover, all LSCO samples showed excellent electrocatalytic performance for the oxygen evolution reaction (OER) process (overpotentials of only 265-285 mV at a current density of 10 mA cm^-2) with minimal η₁₀ for LSCO-900. Based on the experimental data, it was concluded that the formation of a dense amorphous layer on the surface of the particles ensures high stability as a catalyst (at least 24 h) during electrolysis in 1 M KOH electrolyte.