The utilization of sustainable water electrolysis for hydrogen production is currently limited by sluggish kinetics of the oxygen evolution reaction (OER). The development of stable, highly active, and cost-effective OER catalyst supports would facilitate commercialization of water electrolysis technologies. In this study, we report for the first time the use of metallic cobalt nitride (Co4N) nanofibers (NFs) as highly stable and conductive scaffolds for supporting Ir nanoparticles (NPs). The Ir catalysts supported on Co4N NFs exhibit high OER activity and stability in alkaline media as compared with Ir catalysts supported on other materials (Co3O4 or carbon NFs) and commercial Ir/C. These results are attributed to the (i) efficient charge or mass transport between Ir NPs and metallic one-dimensional (1D) Co4N NFs or interfibers, (ii) maintenance of the active oxidation state (Ir3+) of Ir NPs induced by synergistic charge compensation between catalyst and support during OER, and (iii) long-term stability induced by strong metal support interactions.