A blown head gasket can cause oil on spark plugs for a few reasons. First, if the head gasket is blown, it can allow oil to seep into the combustion chamber. This can happen if the head gasket is damaged or not properly installed. When this happens, oil can get onto the spark plugs and cause them to misfire. Second, a blown head gasket can also cause coolant to enter the combustion chamber. This can happen if there is a hole in the head gasket or if it is not properly sealed. Coolant on the spark plugs can cause them to misfire as well. Finally, a blown head gasket can cause exhaust gases to enter the combustion chamber. This can happen if there is a hole in the head gasket or if it is not properly sealed. Exhaust gases on the spark plugs can cause them to misfire as well.