Heat olive oil in a pan over medium heat. Add onion and cook until softened, about 5 minutes.

Add garlic and cook for another minute until fragrant.

Stir in chopped tomatoes, tomato paste, oregano, salt, and pepper. Bring to a simmer and cook for 10 minutes, stirring occasionally.

Gently place fish fillets into the sauce. Cover the pan and cook for 5-7 minutes, or until the fish is cooked through and flakes easily with a fork.

Serve immediately.