The design of cost-effective Pt group metal (PGM)-free nanostructured electrocatalysts with large surface area and easily tunable composition is of pivotal importance to overcome the limitations due to the sluggish kinetics of the oxygen evolution reaction (OER) and, thus, make water electrolysis a commercially sustainable technology. Spinel-type transition metal (TM) oxides have shown great potential as a sustainable alternative to PGM-based catalysts [1]. According to the current assessments on spinel-type catalysts [1,2], the occupation of octahedral sites by redox-active centers and covalence of the metal-oxygen building blocks are mostly responsible for their good electrochemical performance. Hence, many efforts have been devoted to the engineering of the octahedral redox-active TM centres in spinel-structured catalysts [1,2]. High-entropy oxides (HEOs) with multiple TM-cation sites lend themselves to engineering of the octahedral redox-active TM centres [3] to enhance the catalyst reactivity. Their electrochemical performances are comparable or even superior to those of common benchmark catalysts [4-6]. Also, defect engineering has recently emerged as a viable strategy to improve the electrocatalytic performance of nanomaterials [7]. This work focuses on the preparation of defect-rich spinel-type electrospun HEO nanofibers (NFs) and their evaluation as catalysts for OER in alkaline medium. Different equimolar TM combinations were considered, and the NF calcination temperature was varied in the 400800 °C range. The complex and interdependent changes induced by the variation of HEO composition and calcination temperature on the morphology of the fibers, crystallinity of the oxide, density of defects and cation distribution in the lattice, and the effects that these changes produce on the electrocatalytic behavior of the NFs were investigated by a combination of analytical techniques including broadband electric spectroscopy (BES). BES is a powerful dynamic technique to study the influence of nanostructural features on the electric behavior of a material. It allows revealing possible nanoscale heterogeneities, such as nanodomains and their interfaces, analyzing in detail the relaxation modes of the structural network, investigating the interdomain conductivity pathways [8].