Oven cleaner will do a great job on cast iron. If you use it you need to get the "extra strength" stuff. If you use easy off it is the can with the yellow lid. Just make sure the can you buy contains Lye. That is the ingredient needed to strip the skillets of the grease and grime. If you plan on doing a bunch it is easier to buy a can of Lye and add it to water and soak them in it. I have a large rubber maid tub in my garage that has the Lye in it. You can soak hundreds of skillets in it before you need to change it out. Just be careful when handling Lye.
Also as mentioned before if you use vinegar you need to monitor it, if left in too long the skillet will be damaged by the vinegar.