EO is ethyl oleate.
It can be used to replace an oil or can be used in addition to it. It has a very low viscosity so it is very painless and it can be used to make solutions in higher concentrations as it is generally a better solvent. However, it is pretty expensive so most people, at least that I know of, don't use it.
I heard a while back it is also better at preserving steroids and keeping them from expiring but idk if that's bullshit.