This would depend on the type of vehicle, we need a little more information to really be able to tell but the most common problem is leaking spark plug tube seals which allow oil to leak from the valve cover onto the plugs.

I see Barnes got two thumbs down. Sorry, but I agree with him. Either oil is getting past the rings or going down the valve guides.

And anyone who wants to thumbs me down - that's fine, you all are entitled to your opinion. The only OTHER way oil can be on the spark plugs is if someone poured oil into the carb or throttle body, or a blown head gasket.

Hope this helps.

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You have leaking spark plug tower seals. You need to take the valve cover off and replace them. If the plug wire or coil boots are swollen because of the oil they need to be replaced too.

most of the time for sure its the Rings around the pistons big job to fix it.If the oil is on the outside its the valve cover gasket or a loose spark plug...thats it

Oil rings plugged, oil ring return holes in the pistons plugged. Compression ring lands dirty. To clean, switch your motor oil to Mobil1

Inside- worn out valve guides and seals. Outside- leaking valve cover gasket.

Worn cylinder bores and/or oil control rings. Worn valve guides and seals will also let oil into the cylinders.

Blown head gasket,