I have heard that most honey sold in the stores is cut with corn syrup, but I had NO idea that olive oil is often adulterated, too!
I read this post by Cheeseslave, that details how olive oil manufacturers are cutting their oil with other oils, possibly soybean oil, hazelnut oil, and who knows what other oils.
One way to test your oil is to place it in the refrigerator and see if it solidifies. A true olive oil should become solid. If your olive oil doesn't solidify, it could mean that it has been cut with other oils, OR had the waxy portions of the olive oil removed. Read the post by Cheeseslave and also the comments, too. Someone mentioned that Bragg brand didn't solidify when cold. I think that Bragg is a great brand overall and they are probably selling pure olive oil, but they are just removing the waxy portion of the oil. (I personally want my oil as unadulterated as possible, including all the components of the first pressing.)
I started buying Star brand from Walmart because it was labeled "organic". Well, I put it in the refrigerator and it did not solidify. I knew it had to be too good to be true. It was such a great price. I will be looking elsewhere now.