The oil is just the suspension that the progesterone is delivered in. It's the same progesterone in either shot, so it has the same effect. The EO is just a thinner substance than sesame oil, so it's easier to inject. I'm not saying it's easy, but it's comparatively easier. Also, they try it for folks with allergies to sesame/peanut/olive or whatever other oil they try to put in the solution. I was allergic to it all, so I ended up with crinone after 8 weeks on the injections.