Misfire from oil in sparkplug?

I had a misfire and found oil has been going into one sparkplug. I spent the day cleaning and changed all the sparkplugs. There was one covered in oil so I thought...job done, but the misfiring hasn't changed. I'm a bit baffled. I can't see down into the cylinder of my Renault Megane due to the design. The plug is now dry of oil, but I wonder if there is still oil in the cylinder. Any ideas...anyone...Much appreciated.