Cut eggplants into 3/4" cubes and place in 4-quart casserole. Add onion, bread crumbs, oregano and tomato sauce; stir well. Cover tightly and bake at 400 for 45 minutes. Stir vegetables thoroughly, recover and continue to bake until vegetables are very soft when pressed (about 45 more minutes); check occasionally and add water if casserole appears dry.
Spoon ricotta mixture over hot mixture; sprinkle with cheddar cheese. Continue to bake, uncovered, until cheese is melted (about 10 minutes.)
