The concept of a platinum-monolayer electrocatalyst, a heterostructure with a platinum-monolayer shell surrounding a sub-surface transition-metal core chosen to strain or electronically modify the overlayer shell to maximize the electrocatalytic activity, has profoundly influenced the design of the oxygen-reduction electrocatalysts in the past decades. We present a strategy to translate this concept to non-platinum electrocatalysts by using advanced deposition technologies enabled to control the surface and sub-surface chemistries in transition-metal oxides. Starting from RuO₂(110) films epitaxially grown on TiO₂(110) substrates, we manipulate the surface chemistry by controllably deposit Ru_1-xTi_xO₂ overlayers on RuO₂, effectively creating a Ru_1-xTi_xO₂/RuO₂(110) heterostructure. The high quality of the oxide films grown using molecular-beam epitaxy affords the ability to extract the oxygen electro-adsorption energetics, consequently allowing us to quantify the influence of the surface and sub-surface layers on the electro-adsorption and electrocatalytic activity. Putting together the adsorption energetics allows us to test the mechanism for the oxygen evolution reaction (OER) on RuO₂(110) and Ru_1-xTi_xO₂/RuO₂(110) electrocatalysts. We will discuss the implications of these insights in the context of how to find a combination of surface and sub-surface layering to design and create new OER electrocatalysts.