1. Rinse tuna steaks and pat dry. Sprinkle lightly all over with salt and pepper. Pour oil into a 10- to 12-inch nonstick frying pan over high heat. When hot, add onion and stir frequently until limp, about 5 minutes.
2. Push onion to side of pan and add tuna steaks. Cook, turning once, just until browned on both sides, 3 to 4 minutes total. Stir in crushed tomatoes, wine, vinegar, capers, and oregano.
3. Reduce heat to maintain a simmer, cover, and cook until tuna is no longer pink in the center (cut to test), about 15 minutes. Transfer tuna to plates and top equally with sauce.