Was the oil on the tip (part that goes into the combustion chamber) or around the part that you put the plug spanner on?
If it's the first, then it could be worn rings or other engine wear. If it's the 2nd, then rocker seals as said above.
If you're using water you may have a leak in anyone of the umpty million hoses in the water system. 7 of them are hidden on the back of the engine. It's possible you may have a head gasket on the way out or a leaky radiator. It might be worth getting the cooling system pressure tested to try and find the cause.