I doubt the head is cracked. A cracked head or blown gasket does not usually allow oil into the combustion chamber.

Rings and/or valve guides and seals will.

Is there oil on the plugs (below the threads) or above, on the insulator? If it is the later, you may have a bad valve cover gasket or spark plug tube seals. This will allow oil into the boot area, then when you pull the plugs, it runs down to the electrode end of the plug.

Good Luck,
Michael