Low temperature electrolysis plays a critical role in our effort for energy generation with net zero or negative carbon impacts. This process produces green hydrogen through catalytic splitting of water and requires the development of highly active and, perhaps more importantly, highly stable electrocatalysts for oxygen evolution reaction (OER). This presentation will focus on examining the structure-property relationship of complex oxides that are made of A_xB_yO_z, where A and B can be a single metal cation or mixed cations, respectively. Depending the compositions, perovskite, pyrochlore, and Ruddlesden-Popper (RP) phase solids all have been found to be highly active for OER in either acid or base conditions. Noticeably, structurally defective solid catalysts such as those observed in defect perovskite (J. Am. Chem. Soc., 2014, 136, 14646-14649) often possess high activity. I will review some of the latest advancements in understanding the mechanisms of OER and discuss the new results on regulating the cation sites and oxygen defect chemistry for enhancing the bond-stability of key catalytic constituents and surface structures for OER performance.