In a large skillet, heat olive oil over medium heat.
Add the sliced bell peppers and onion to the skillet and cook for 5 minutes, until softened.
Add the minced garlic, dried oregano, salt, and black pepper to the skillet and cook for an additional minute.
Push the vegetables to one side of the skillet and add the calamari rings to the other side.
Cook the calamari for 2-3 minutes on each side, until opaque and cooked through.
Squeeze the juice from the lemon slices over the calamari and vegetables.
Serve the calamari and bell pepper medley hot as an appetizer or over rice or pasta as a main dish.