Heat 2 tablespoons of the oil in a 10-inch cast-iron skillet over medium-high. Add onion; cook, stirring occasionally, until softened and beginning to brown, 3 to 4 minutes. Add garlic; cook, stirring constantly, 1 minute. Add tomatoes, 2 tablespoons of the basil, 1 teaspoon of the salt, and 1/2 teaspoon of the pepper; cook, stirring occasionally, until tomatoes burst and release their juices, 6 to 7 minutes. Continue to cook, stirring occasionally, until sauce thickens slightly, 1 to 2 minutes. Transfer to a bowl; cover and keep warm.
Wipe skillet clean. Pat scallops dry with paper towels, and season with remaining 1/4 teaspoon each salt and pepper. Heat remaining 2 tablespoons oil in skillet over high. Add scallops, and cook until golden brown, about 1 minute and 30 seconds per side. (Do not overcook.)
Cook orzo according to package directions; drain. Stir in butter and parsley.
Divide orzo among serving plates; top each with about 2/3 cup tomato sauce and 4 scallops. Sprinkle evenly with remaining 2 tablespoons basil.