From what I have read, essential oils *are* oils. They are just a special kind of oil called volatile aromatic compounds, which simply means that they change from their liquid state to gas easily. Essential oils by themselves can stain fabric:
https://answers.yahoo.com/question/i...1230924AAq3je9
On the other hand, many quilters make up their own Best Press and use a drop or two of essential oil in the mixture without obvious harm to fabric. Probably the dilution in vodka or witch hazel is enough to prevent staining. Plus the heat from the iron would hasten the oil's change in state from liquid to gas.
Edit: Here is an article that explains what essential oils are:
https://doterra.com/US/en/what-is-an-essential-oil