there is oil in one of my spark plugs

Hello, First, you must determine if the oil has drained from the valve cover and soaked the plug as it was coming out. To do this, just clean the plug, clean the area on the head surrounding the plug, and run the engine. If you get no oil, but there is an obvious leak on the valve cover, that is the answer. If you get oil on the plug again, you likely have a broken ring or bad valve seal draining oil into the cylinder.