I did decrotive concrete for a few years and prior to laying down any coatings on driveways, I would use one of those propane torches that connect up to a small propane tank. The kind you use for burning weeds around a small place. I would apply enough heat to get the oil stain smoking then back off a little so not to get the concrete to hot. When it stopped smoking, the oil was gone. Well, at least 95 percent of it. I wonder if bleach were applied after this process, would the concrete look almost oil stain free? Just a thought.