It has been shown that nanofluids can significantly impact the forces at both fluid-fluid and fluid-solid interfaces. Nanofluids have also been reported in laboratory studies to enhance oil recovery. However, mechanisms of oil recovery by nanofluids in natural porous media are not well understood. More specifically, there is an absence of direct and quantitative evidence of oil mobilization by nanofluids at the pore level. In this study, we probe the mechanisms through which nanofluids enhance pore-scale sweep efficiency during oil displacements. We decouple the effects of nanofluids on interfacial tension (IFT) and wettability by using a combination of silica nanoparticles and a nonionic surfactant. We performed a set of miniature core-flooding experiments integrated with a high-resolution imaging technique at elevated pressure and temperature conditions to examine the effects of nanofluids on oil recovery in Berea sandstone samples aged dynamically with crude oil. The pore-scale displacement mechanisms were investigated by directly measuring in situ contact angles in visualized images of the pore space, mapping the distribution of remaining oil globules, and examining the dynamic IFT data. The simple SiOx nanofluid had better performance in recovering oil from an oil-wet sandstone sample compared with blank brine. Based on in situ contact angle measurements, wettability reversal was identified as the principal mechanism responsible for the observed behavior. The complex SiOx nanofluid, which incorporated a nonionic surfactant to lower IFT, generated the highest oil recovery because of a synergistic effect between the wettability reversal and IFT reduction mechanisms.