Abstract: Multicomponent Gd_1−xSm_xBa_0.5Sr_0.5CoCuO_5+δ double perovskites are optimized for application in terms of chemical composition and morphology for the use as oxygen electrodes in solid oxide cells. Structural studies of other physicochemical properties are conducted on a series of materials obtained by the sol–gel method with different ratios of Gd and Sm cations. It is documented that changing the x value, and the resulting adjustment of the average ionic radius, have a significant impact on the crystal structure, stability, as well as on the total conductivity and thermomechanical properties of the materials, with the best results obtained for the Gd_0.75Sm_0.25Ba_0.5Sr_0.5CoCuO_5+δ composition. Oxygen electrodes are prepared using the selected compound, allowing to obtain low polarization resistance values, such as 0.086 Ω·cm² at 800°C. Systematic studies of electrocatalytic activity are conducted using La_0.8Sr_0.2Ga_0.8Mg_0.2O_3−δ as the electrolyte for all electrodes, and Ce_0.8Gd_0.2O_2−δ electrolyte for the best performing Gd_0.75Sm_0.25Ba_0.5Sr_0.5CoCuO_5+δ electrodes. The electrochemical data are analyzed using the distribution of relaxation times method. Also, the influence of the preparation method of the electrode material is investigated using the electrospinning technique. Finally, the performance of the Gd_0.75Sm_0.25Ba_0.5Sr_0.5CoCuO_5+δ electrodes is tested in a Ni-YSZ (yttria-stabilized zirconia) anode-supported cell with a Ce_0.8Gd_0.2O_2−δ buffer layer, in the fuel cell and electrolyzer operating modes. With the electrospun electrode, a power density of 462 mW·cm⁻² is obtained at 700°C, with a current density of ca. 0.2 A·cm⁻² at 1.3 V for the electrolysis at the same temperature, indicating better performance compared to the sol–gel-based electrode.