In a search for new, improved mixed ionic-electronic conductors (MIEC) exhibiting a desired set of structural, thermomechanical, transport, and electrocatalytic properties, various complex perovskites and related oxides have been proposed i.a. as excellent oxygen electrode materials for Solid Oxide Cells (SOCs). Apart from their relatively complex and often highly anisotropic structural features, usually manifested through formation of various superstructures, such oxides may also show compositional complexity, with the emerging group of the high entropy materials being developed as well. In this work several systems are described in more details, including the A-site layered RE(Ba,Sr)Co_2-yMn_yO_5+δ (RE: selected rare-earth cations; 0 ≤ y ≤ 2), for which, depending on the Mn content, the physicochemical properties can be optimized for the SOC, or alternatively, for oxygen storage-related applications [1, 2]. In another group, RE(Ba,Sr)Co_2-yCu_yO_5+δ, doping with copper is found to occur in a limited range, however, materials at the limit of formation of solid solutions may exhibit enhanced electrocatalytic properties [3]. Together with the appropriate other characteristics, as well as significantly decreased cobalt content, they appear as very good candidates for SOC oxygen electrodes. Importantly, depending on the chemical composition and preparation method (e.g. by electrospinning), some of those oxides can be obtained in a single perovskite structure, showing altered properties in comparison to their respective double perovskite counterparts. Also, pure Cu-based perovskite-related oxides with a general formula of La_1-x(Ba,Sr)_xCuO_3-δ are found as very promising candidates for SOC oxygen electrodes, however, possibility of obtaining single phase compounds is relatively limited in the considered system, and often, the oxygen sublattice shows strong ordering, which hinders the ionic conductivity [4]. In this aspect, finding systems with the (partially) disordered oxygen vacancies is reported as beneficial, especially regarding electrocatalytic activity at moderate temperatures [5]. In another approach, multicomponent at either the A- and/or B-site perovskites have recently become of interest, however, the actual role of the high entropy in terms of influencing the properties (in terms of variation from the expected solid solution-based rules) is not yet understood [6]. Finally, the (small RE cation-based) REMnO₃-type hexagonal oxides, with compositions close to the perovskite-hexagonal phase transition border, can be optimized for the temperature swing absorption processes, which enable oxygen separation from air at relatively low temperatures of 200-300 °C [7]. The oxides show unique set of the physicochemical properties, including presence of interstitial oxygen defects. This work is summarized with several guidelines, which should be helpful for designing and obtaining such complex oxides exhibiting the desired useful properties.