Lightly coat the inside of the slow cooker with oil or cooking spray to prevent sticking.
Crack the eggs into a mixing bowl, add the milk, salt, pepper, and oregano, then whisk until smooth.
Stir the diced bell pepper, spinach, onion, and mushrooms into the egg mixture.
Pour the combined ingredients into the prepared slow cooker.
Scatter the crumbled feta evenly across the surface.
Cover the slow cooker and cook on low until the center is set and a knife inserted comes out clean.