The understanding and control of the surface reactivity of the oxygen-electrode materials is a key enabler for the efficiency and durability of solid oxide fuel and electrolysis cells (SOFC / SOEC) at intermediate temperatures. The bulk electronic and ionic conductivities of the electrode materials are well defined traditionally by electrochemical methods. However, it is now increasingly realized that surface structure and chemistry govern the reaction and transport mechanisms and kinetics, and that they are not static - they dynamically respond to their surrounding harsh environments and age over extended periods. The surface atomic and electronic structure at the functional conditions of SOFCs is still not well explored because of the harsh conditions of temperature and reactive gases that make it difficult to access with traditional surface science techniques. We develop and use state of the art in situ surface science probes and first principles based computational simulations to understand and control the effect of heterogeneities such as defects, chemical segregation layers, and strained hetero-interfaces on the oxygen reduction reaction on the basis of the governing surface electronic structure.