Heat 1 Tbs. of the oil in a 12-inch nonstick skillet over medium-high heat. Season the fish on both sides with 1/2 tsp. salt and 1/4 tsp. pepper. Cook, turning once, until the fish is just opaque throughout, 3 to 5 minutes per side. Transfer to a plate and keep warm.
Heat the remaining 1 Tbs. oil in the skillet over medium heat, add the onion and cook, stirring occasionally, until softened, about 3 minutes. Add the garlic and cook until fragrant, 30 seconds more. Stir in the wine and oregano. Bring to a boil, lower the heat to low, and simmer until the liquid is reduced by half, about 3 minutes.
Stir in the tomatoes and their juice, the olives, and capers. Bring to a simmer and cook until the sauce has reduced slightly, about 5 minutes. Stir in the parsley. Serve the fish topped with the sauce.