Complex oxides comprised of multiple positively charged metal cations exhibit a host of intriguing and useful properties for new technologies. Perovskite oxides with the chemical formula ABO₃ and spinel oxides with the formula AB2O₄ have some of the richest behavior. These materials may be metallic, semiconducting, or insulating, and exhibit ferroelectricity, with a built-in electric polarization, ferromagnetism, or superconductivity. This combination of properties in a single class of materials offers rich opportunities for engineering of unusual combinations of behavior through the design of multi-layer thin film materials. Through the use of molecular beam epitaxy (MBE), we are able to engineer these materials down to the atomic level so that interfaces between two different materials can be controlled to produce desirable properties. In this talk I will present two examples of this type of interfacial engineering, showing how we can design, model, and characterize these properties through a wide variety of techniques. I will first discuss our work on spinel and perovskite oxide nanocomposites that can be used in the oxygen reduction and oxygen evolution reactions. Using a combination of x-ray photoelectron spectroscopy (XPS), x-ray absorption spectroscopy (XAS), scanning transmission electron microscopy (STEM), and spectroscopic ellipsometry we have answered fundamental questions about the properties of CoMn₂O₄ and MnFe₂O₄. Ongoing work focuses on integrating these materials with perovskites such as LaNiO₃ and LaFeO₃ to produce bifunctional catalysts. Our second project focuses on the synthesis of defect-free SrTiO₃ and Sr(Ti,Nb)O₃ thin films using hybrid MBE. Using XPS surface studies, we have answered fundamental questions regarding this emerging growth technique. Ongoing work focuses on the use of these materials to produce novel oxide heterostructures for topological phases, and spintronic devices.