Pat the cod dry with a paper towel, and season with ½ teaspoon salt and ¼ teaspoon black pepper on both sides; set aside.
In a large heavy skillet, heat the olive oil on medium heat. Add the chopped onions and cook until soft and translucent, about 3-5 minutes. Add the garlic and stir until fragrant, about 1 minute.
Pour the crushed or diced tomatoes, season with the remaining salt and pepper along with oregano and crushed red pepper. Bring the mixture to a boil, then reduce the heat to low and simmer the sauce for 10 minutes.
Add the capers and olives to the tomato broth. Carefully nestle the seasoned pieces of cod into the sauce without covering the fish with the sauce. Cover with a tight-fitting lid or foil until the fish is cooked through, about 8-10 minutes.
Serve on its own or with quinoa, pasta, rice or potatoes to soak up the sauce.