Iridium is widely recognized to be the best electrocatalyst in terms of a balanced activity and stability for the oxygen evolution reaction (OER) of water splitting in acidic media, while the activity is still insufficient for the PEM electrolysis technology to be commercially viable. In this study, we report a promising approach to drastically enhance the activity of Iridium retaining the excellent stability nature; electrochemically activated Ir oxide black (A-IrOx-B) prepared by the selective etching of less noble component from the binary oxide. Binary IrOs oxide black (IrOsOx-B) were formed by using a solvent-free method, and then an electrochemical activation was performed by the selective etching of Os oxide in order to obtain the activated Ir oxide black possessing high surface area. It was interestingly revealed that the OER activity of A-IrOx-B surpassed that of IrOx black, and approached that of state of the art surface segregated IrRu oxide (S-IrRuOx-B). Most notably, the OER activity of S-IrRuOx-B decreased after stability testing because of the dissolution of Ru, while the activity of A-IrOx-B changed little and eventually surpassed the activity of S-IrRuOx-B.