Liquid interfaces are dynamic environments characterized by chemical and physical properties that are different from the bulk. Here we use ultrafast, two-dimensional infrared spectroscopy and molecular dynamics simulations to measure the effect of oil phase viscosity on interfacial water dynamics in reverse micelles. The oil and water phases are separated by a 2.3-3.0 nm surfactant interface. Increasing the oil viscosity from 0.3 to 10.5 cP slows down water motions from 0.9 to 1.5 ps, as measured by the carbonyl frequency fluctuations. Simulations, which agree semi-quantitatively with experiments, show that water self-diffusion at the interface slows with increased oil viscosity. This study shows that the oil phase "transmits" its dynamics to the aqueous phase through the surfactant layer with minimal perturbations to surfactant-water interfacial structure.