The oil may be a problem, but that's a separate issue. To flatten the stone you don't need another stone, although some people just use the side of a concrete block. Get some wet/dry sandpaper in assorted grits, probably starting around 100 grit, and attach them to a piece of glass or a 12x12 piece of marble/granite tile with some spray adhesive, and then scrub your stone on that while flushing the sand paper liberally with water (like in a sink while you're doing it). Work your way up to the finer grit papers and end up around 220. Give the stone a test sharpening something, and if there's a substantial improvement you'll know if the side of the stone will be salvageable.