Preheat the oven to 350°F (175°C).
Cook the breakfast sausage in a skillet over medium heat until browned and crumbly, then drain excess fat.
In a large bowl, whisk together the eggs and milk until well combined.
Add the diced bread, cooked sausage, bell peppers, onions, oregano, salt, and pepper to the egg mixture, and stir gently to combine.
Pour the mixture into a greased 9x13 inch baking dish, spreading it evenly.
Sprinkle the shredded cheese evenly over the top.
Bake in the preheated oven for 45-55 minutes or until the center is set and the top is golden brown.
Allow the casserole to cool for 10 minutes before slicing and serving.