There are various calibration solutions available for ORP meters, although they are rather unpredictable.
Don't trust that stuff myself. So I mix it fresh every time. Good to the exact mV by temp and pH.
http://www.amazon.com/Sensorex-B125-Piece-Calibration-Solution/dp/B006WC16DU
I agree that something other than ORP would be a better choice for detecting ozone output. Ozone does have a distinctive odor, although it can damage all the various mucous membranes, so I'd be careful with it.
I just use the ol' olfactory. But short of that you could use KI. Potassium Iodide, or nuclear radiation doomsday pills. Crush one between two spoons, mix that in a ounce of water. Wet a paper towel in it. Ozone will turn it brown instantly. I use it to look for leaks by wrapping the paper towel around fittings.
http://www.amazon.com/Natures-Plus-...=sr_1_14?s=hpc&ie=UTF8&qid=1384791229&sr=1-14
--John