In a large pot, heat the olive oil over medium heat. Add the diced onion and sauté until translucent, about 5 minutes.
Stir in the minced garlic and diced bell pepper, cooking for an additional 2-3 minutes until fragrant.
Add the diced tomatoes, fish stock (or clam juice), white wine, dried oregano, and red pepper flakes. Bring the mixture to a simmer.
Once simmering, add the shrimp, mussels, clams, and white fish. Cover the pot and cook for about 5-7 minutes, or until the mussels and clams have opened and the shrimp are pink and opaque.
Season with salt and pepper to taste. Remove from heat and let sit for a couple of minutes.
Serve hot, garnished with fresh parsley and accompanied by crusty bread for dipping.