[header] How to kosher an oven [title] Use a caustic oven cleaner. [step] Begin the koshering process by spraying a caustic oven cleaner inside of your oven. Leave the oven on for as long as the instructions recommend after you've finished spraying the oven with the cleaner. 
The cleaner will loosen up the hardened grease. Scrape the oven with steel wool once the oven has cooled down.