Oven grease is difficult to get rid of and needs a lot of effort. But keeping a pot of hot water and a bowl of ammonia overnight will loosen it and make cleaning easier, according to the One Good Thing By Jillee blog.
Heat the oven to 65°C and turn it off. Place a pot of boiling water on the bottom rack. On the top rack, put a heat-safe bowl or baking dish containing 1 cup of ammonia. Shut the oven door and leave it overnight.
The next morning, open the oven and remove both the bowl of ammonia and the pot of water. Don't dispose of the ammonia; you'll want to use it later. Remove the racks and leave the oven door open to air out for 15 minutes. Add 1-2 teaspoons of liquid dish soap to the ammonia, along with a quart of warm water, and using a heavy-duty nylon scrubbing pad dipped in the ammonia mixture, begin to wipe away the softened grease and grime along the sides and bottom of the oven. It should be a fairly easy job at this point.
Needless to say, when working with ammonia, it's always advisable to use kitchen gloves. This trick is similar to how you clean stove burners, but Jill notes that the ammonia did not have a powerful odour after having sat in the oven overnight.
How To Easily Clean Your Oven [One Good Thing By Jillee]