The success of foam displacement in porous media largely depends on its stability, which is adversely impacted by the presence of oil. In this study, we present the results of an experimental investigation into the effect of oil type and saturation on foam rheology in Berea sandstone using the nuclear magnetic resonance imaging technique. The results of foam quality scan in the presence of remaining hexadecane was compared with those from the oil-free case. We showed that the calculated apparent viscosity values in the presence of remaining hexadecane were higher than those in the absence of oil except at very high foam qualities. This was attributed to the dominance of relative permeability reduction and generation of oil-in water emulsions over the foam-weakening effect of oil. A closer analysis of the fluid distribution further allowed us to quantify the relative significance of the identified oil displacement mechanisms, namely, the increased capillary number, and the micellar solubilization. Additionally, we correlated the foam strength with the oil saturation from the foam-oil co-injection tests. Because of the detrimental effect of oil on foam stability, it is found that the apparent viscosity first decreases with increasing oil (hexadecane and octane) saturation; then, due to oil emulsification, apparent viscosity increases with oil saturation (or more strictly with the oil fractional flow). Compared to octane, hexadecane resulted in less foam destabilization and higher oil emulsification. The co-injection experiment with crude oil exhibited a similar behavior to octane. Findings from this study suggest that, despite the destabilizing impact of oil on foam, the resulting steady-state apparent viscosities could be much larger than the viscosity of the in situ oil. This leads to a favorable mobility ratio during the oil-displacement process.