Preheat oven to 400 degrees. In a food processor, pulse tomatoes until coarsely chopped. In a large ovenproof skillet, heat oil over medium-high. Season chicken with salt and pepper and cook, skin side down, until golden and crisp, about 7 minutes. Flip and cook 1 minute. Transfer chicken to a plate and drain all but 1 tablespoon fat from skillet. Reduce heat to medium. Add onion to skillet and cook until soft, about 5 minutes. Add garlic and cook until fragrant, 1 minute. Add tomatoes, season with salt and pepper, and bring to a boil.
Add chicken, skin side up, and oregano, then transfer skillet to oven. Cook until chicken is cooked through, 25 to 30 minutes.