In a large skillet over medium-high heat, add 4 tablespoons oil. Once oil is hot, cook onions and garlic until sizzling, about 2-4 minutes. Add pepperoncino and cook for 1 minute. Add tomatoes, oregano, red pepper, and salt, then bring to a simmer and cook uncovered to slightly thicken sauce. Stir calamari into skillet and simmer covered for about 30-40 minutes (less if your calamari was small) until just cooked through.