This study investigates the effect of different oil concentrations on the critical micelle concentration of the following surfactants: sodium dodecyl sulfate as an anionic surfactant, Tween 80 as a nonionic surfactant, and cetyltrimethylammonium bromide as a cationic surfactant. The critical micelle concentration was determined from abrupt changes in solution turbidity and specific electrical conductivity. The addition of oil (10–50 mg/L) was shown to alter the critical micelle concentration. For sodium dodecyl sulfate, the critical micelle concentration decreased at low oil concentrations and increased at higher oil concentrations. For cetyltrimethylammonium bromide and Tween 80, a consistent shift of the critical micelle concentration toward higher concentrations was observed. The results indicate that oil may act as a co-surfactant or hydrophobic additive influencing micellization processes. Specifically, oil can increase the surfactant concentration required to initiate self-assembly by stabilizing oil-in-water emulsions and modifying the microenvironment of polar head groups (for cetyltrimethylammonium bromide, Tween 80, and sodium dodecyl sulfate at moderate and high oil loading rates). The following practical recommendations are formulated: (1) engineering calculations of surfactant dosages for hydrocarbon solubilization and microemulsion formation should account for a potentially nonmonotonic critical micelle concentration response at low to moderate oil contents, particularly in anionic systems; (2) for ionic surfactants, the ionic strength and hardness of water should be monitored to avoid “disguising” the oil-induced critical micelle concentration shift; and (3) for nonionic ethoxylated surfactants, the presence of oil may increase the surfactant dosage required to achieve micellization. The findings are of practical significance for both the development of bioremediation technologies for oil-contaminated ecosystems and enhanced oil recovery.