Oil on the sparkplug can only be in one of two places. Above the combustion chamber (where sparkplug wires connect to the plug itself) or inside the chamber where the electrode resides. We assume you're talking about the former. In that case oil is leaking from the seal that seals the spark plug tube to the valve cover. If this is the case which most likely it is, from the rubber seals hardening over age, you remove the valve cover. You remove the plastic alternator wire casing (one bolt) and move it aside. Remove the two vacuum hoses from the valve covers and the 3 bolts on top along with their rubber grommet seals. You might have to tap the cover with a mallet or pry it with a screwdriver to loosen it. It's a simple procedure. The hardest part will be removing the sparkplug seals from the cover itself. I have found over the years, the easiest way to remove the hardened seals is to use a large flathead screwdriver from the inside and place the blade in between the cover and seal and pry it towards you. Gently pry in about 4 places and it should pop right out. Do not use pliers or vise grips as it will tear. Use a dowel or socket the diameter of the seal and gently tap in the new seals.
If you buy a valve cover gasket kit, it should include the three grometts, 4 SP seals and the gasket itself. Should be about $25-30 from your local auto parts store. Also apply a small dab of RTV sealant in the 90 degree vertical corners (4) of the mating area on the cylinder head itself.
If you have oil on the electrode of the SP itself, that's a whole another issue dealing with valve guide seals or piston rings.