BaTaO2N responds to visible light up to 650 nm and exhibits photocatalytic activity during the hydrogen and oxygen evolution reactions. However, the difficulty in promoting both processes simultaneously hinders the realization of overall water splitting (OWS) over this material. Herein, we demonstrate the activation and stabilization of BaTaO2N-based photocatalysts for visible-light-driven OWS by combining flux-assisted nitridation with Mg doping and co-loading of Cr2O3/(Na)Rh and IrO2 co-catalysts. RbCl-assisted nitridation produced single-crystal BaTaO2N particles doped with Mg and so promoted charge transfer to the co-catalysts. The coloading of Cr2O3/(Na)Rh and IrO2 at moderate temperatures as co-catalysts to promote the hydrogen and oxygen evolution reactions, respectively, allowed the BaTaO2N photocatalyst to evolve hydrogen and oxygen simultaneously from water. This work highlights the importance of carefully tuning semiconducting and catalytic properties to realize effective charge transfer when preparing narrow-bandgap oxynitrides for OWS.