High-level control of atomic and surface structures is a hallmark of the application of nanomaterials in a range of electrochemical and electrocatalytic devices, such as water electrolyzer. They play critical roles in our effort to develop energy conversion and storage technologies that have net zero carbon impacts. Nanostructured metal oxides made for catalyzing the oxygen evolution reaction (OER) is one representative example. Unlike the traditional heterogeneous catalysis, both bulk and surface properties are important in the design of active and durable electrocatalysts. This is because besides the adsorbate evolution mechanism (AEM), lattice oxygen mechanism (LOM) is often involved in the catalytic cycle. In this talk, I will present our recent work on the synthesis-structure-electrocatalytic property relationship of complex oxides that can be described in a generic formula of A_xB_yO_z, where A and B can be a single metal cation or mixed cations located at a given lattice site. We haveexamined several archetypes of oxide structures, including perovskite, pyrochlore, spinel, and Ruddlesden-Popper (RP) phasecompounds and their site-mixed solids, all of which are found to be active for OER under either acid or base conditions. Ourresults indicate defect engineering in these solids is particularly important for OER catalysis. Thus, it is essential, besides agood understanding of heterogeneous catalysis, one needs to take a solid state chemistry view in order to uncovering thecatalyst design for optimal performance. How to regulate the cation sites and oxygen defect chemistry for enhancing the bondand lattice stability of key structural constituents can be important. The new understandings should inform the approach tothe fabrication of earth-abundant oxide electrocatalysts for hydrogen production and utilization.