The vertical resistance of Cu/Ta/Ru/Cu stacks is calculated using a combination of first-principles density functional theory (DFT) and a Non-Equilibrium Green's Function (NEGF) formalism. The effects of oxidizing either one or both of the Ta and Ru layers are analyzed. These oxides can be either metallic (TaO and RuO2) or insulating (Ta2O5) in nature. Simulations indicate that for the metallic oxides, the presence of RuO2 results in more electron scattering than TaO. Complete oxidation of both Ta/Ru layers results in a ≈3× increase in resistance for Cu/TaO/RuO2/Cu relative to the un-oxidized structure, and a ≈8x increase in resistance for Cu/Ta2O5/RuO2/Cu relative to the un-oxidized structure. Electron transmission/reflection coefficients as well as values of total resistance are reported for each interface structure. These results highlight the importance of identifying and controlling oxygen contamination in high-volume manufacturing in order to obtain low resistance vertical interconnects and favorable device performance.