Phase behavior and formation of self-assemblies in ternary water/ penta(oxyethylene) dodecyl ether (C-12-EO5)/amphiphilic silicone copolymer, poly(dimethylsiloxane)-poly(oxyethylene) (SimC3EO3.2) systems, were investigated. These silicone copolymers are very hydrophobic because of the short EO chain and are essentially water-insoluble amphiphilic oils, similar to high alcohols or fatty acids. In general, the surfactant layer curvature becomes less positive upon addition of amphiphilic oils, similar to what happens with high alcohols or fatty acids. This is what is observed for m = 5.8. However, in the ternary water/C12EO5/Si25C3EO3.2 system, the surfactant layer curvature becomes more positive, and micellar cubic, normal hexagonal, and bicontinuous cubic phases are also formed, although a lamellar liquid crystal forms only in the water-C12EO5 system. In the liquid crystal regions, it was found that Si25C3EO3.2 is not dissolved in the thin bilayer of the C12EO5 lamellar phase, whereas the long and bulky Si25C3 chain forms an oil core in those cubic, hexagonal, and bicontinuous cubic phases, in which the surfactant is absorbed at the hydrophobic interface of each aggregate. Then, this opposite effect of added amphiphilic silicone copolymer or oil to the surfactant solution is attributed to the long and flexible Si25C3 chain which makes an oil pool inside the aggregates. This different behavior of small and large SimC3 chains is also observed in the cloud temperatures. In fact, the cloud temperature of C12EO5 aqueous solution decreases upon addition of Si5.8C3EO3.2, whereas it increases by adding a long hydrophobic chain, like Si14C3EO3.2 or Si25C3EO3.2 silicone oil. It was confirmed by DLS that long rod micelles, which are present in the water-C12EO5 system, become small spherical micelles upon addition of Si25C3EO3.2. Consequently, hydrophobic SimC3EO3.2 changes the micellar shape in two different ways depending on the length of the SimC3 chain. For small chains (e.g., m = 5.8), the surfactant-copolymer layer curvature changes to less positive by forming a mixed layer. Long and bulky chains (e.g., m = 14, 25) make an oil core and this tends to make the curvature more positive, being this effect pronounced with increasing the hydrophobic chain length.