In a large skillet, heat the olive oil over medium heat. Add the chopped onion and sauté until it becomes translucent, about 5 minutes.
Stir in the minced garlic and cook for an additional 1 minute until fragrant.
Add the diced tomatoes, olives, capers, red pepper flakes, and oregano to the skillet. Stir to combine and let the mixture simmer for about 10 minutes, allowing the flavors to meld.
Season the fish fillets with salt and pepper on both sides. Carefully place the fillets into the skillet, spooning some of the sauce over the top.
Cover the skillet and cook for 8-10 minutes, or until the fish is cooked through and flakes easily with a fork.
Remove from heat and garnish with chopped parsley. Serve with lemon wedges on the side for squeezing over the fish.