After moving in to a new house, I found my new oven in this condition:
I guess the previous home owners never got around to cleaning it. Therefore, work had to be done before I was going to even consider cooking anything in that mess.
After researching different ways to clean an oven, I found that it was fairly simple. All you need is water and baking soda.
Add about 1/2 cup of baking soda to a bowl.
Mix just enough water to make a runny paste. You do not want this paste to be too thick.
Apply your mixture to your glass and door and wait 30 minutes to wipe it off. If there are still some remaining spots, just scrub them off with a scour pad.