In a large pot, heat the olive oil over medium heat. Add the diced onion and bell pepper, cooking until softened, about 5 minutes. Stir in the minced garlic and cook for an additional minute until fragrant.
Add the diced tomatoes (with their juice), seafood stock, white wine, oregano, basil, and red pepper flakes. Bring the mixture to a simmer and let it cook for about 10 minutes to allow the flavors to meld.
Season the broth with salt and pepper to taste.
Add the shrimp, mussels, clams, and fish to the pot. Cover and cook for about 5-7 minutes, or until the mussels and clams have opened and the shrimp are pink and opaque. Discard any mussels or clams that do not open.
Serve the cioppino hot, garnished with fresh parsley and accompanied by crusty bread for dipping.