This is a great question, and the answer really is "I don't know."
I don't buy cosmetic products from stores, I make them. But I was having trouble with my mascara smearing all over my eyes when I applied it, so I stopped at my local drug store with an awesome cosmetic section and talked to the store clerk. She told me my problem was that I was too oily. (True, but in the end, not the issue.) She recommended an "oil free" product that contained shea butter. Shea butter for oily, acne prone skin? I politely declined the product, then ranted all the way home in the car.
In the end, I wonder if that product wasn't "oil free" because it didn't contain an oil. It contained a butter - which I would argue is a solid oil, but whatever - and esters and fatty alcohols - behenyl alcohol, if I recall correctly - but there were no oils. If you look at any lotions labelled as "oil free", you'll see all those fatty ingredients there.
Neutrogena's Oil free moisturizer for sensitive skin:
Water, Glycerin, Ethylhexyl Palmitate, Dimethicone, Petrolatum, Cyclomethicone, Soybean (Glycine Soja) Sterols , Isopropyl Isostearate, Cetyl Alcohol, PEG 10 Soy Sterol, Glyceryl Stearate, PEG-100 Stearate, C12-15 Alkyl Benzoate, Carbomer, Tetrasodium EDTA, Sodium Hydroxide, Diazolidinyl Urea, Ethylparaben, Methylparaben, Propylparaben
Everything from ethylhexyl palmitate to C12-15 alkyl benzoate is an oil soluble ingredient that can be used in place of oils when we're making lovely creations in our workshops. They are right: There are no oils in this product. There are soybean sterols - which are probably quite lovely if they're anything like Croda's Super Sterol or the olive oil sterols (I think I've written about them, but can't find a link!) - which are phytosterols from soybean. This isn't soy bean oil; it's an ingredient derived from soy bean oil.
Related posts: Can phytosterols penetrate your skin?
The short answer to your question is that there is no definition for "oil free" in a moisturizer, except they don't contain oils. They contain all kinds of oil soluble ingredients and silicones, but no real "oils".