Metal oxides are desirable photoanodes for photoelectrochemical (PEC) splitting of water due to their high stability in aqueous electrolytes and resistance to oxidation. However, most metal oxides have bandgaps that are too large for efficiently absorbing sunlight. In this talk, we show that oxide double perovskites exhibit unusual flexibility for bandgap engineering, due to the multiple choices of metal cations. Taking barium bismuth niobate double perovskite as a case study, density-functional theory calculation (DFT) suggests that the bandgap can be effectively narrowed if the oxide is made Bi-rich and Nb-poor, giving a composition of Ba2Bi1+xNb1-xO6. The excess Bi atoms possess 5+ charge states to maintain overall charge neutrality within the material. As a result, the conduction band minimum is derived from the lower-energy unoccupied Bi 6s states, leading to effective bandgap reduction. Material synthesis confirms the predictions from DFT calculation. We find that with x = 0.4, the Ba2Bi1.4Nb0.6O6 double perovskite oxide produces pure single phase thin films with a narrow nearly-direct bandgap of about 1.64 eV. We further show that photoanodes made of Ba2Bi1.4Nb0.6O6 thin films exhibit promising PEC activity and stability. Our results suggest a new strategy for bandgap engineering of metal oxides towards highly active photoanodes for efficient water-splitting applications.