Heat the olive oil in a large pot over medium heat.
Add the diced onion and bell pepper, sautéing until softened, about 5 minutes.
Stir in the minced garlic and cook for an additional minute until fragrant.
Add the diced tomatoes (with juices), fish stock, white wine, oregano, red pepper flakes, salt, and pepper.
Bring the mixture to a simmer and let it cook for about 10 minutes to allow the flavors to meld.
Add the shrimp, mussels, clams, and white fish to the pot.
Cover and cook for 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.
Stir in the chopped parsley and adjust the seasoning if necessary.
Serve hot with lemon wedges on the side.