What causes an oily smell in scuba air tanks?

Answer 1

That's Easy... ITS OIL!! The compressor which filled the tanks has contaminated the cylinders, if you dive with them you risk serious problems!! they need professional cleaning, especially if you are using Nitrox.

Addition

There is another possibility and that is if new hoses are being used at the fill station. The air could pick up an odor going through them. But I would be very concerned it is oil as stated above. The fill station probably knows what is going on to give an odor, but if they cannot explain it then don't use it. As stated above, you may also want to have your tank visually inspected and cleaned in case any oil vapors did get in there. When in doubt, just don't use it.