Once the older employee dies, grab her glasses and use it on the animatronic thats sitting in the hallway leading to the oven. You'll be able to summon "The Kid" to help with the boss in the oven.
I didn't discover "The Kid" until I had already painfully toughed out the oven battle. Felt a bit like an idiot for having my Waitress take it out on her own, pff.
