Heat butter in medium saucepan over medium heat until melted. Add onion, oregano, and ?½ teaspoon salt; cook, stirring occasionally, until liquid has evaporated and onion is golden brown, about 5 minutes. Add garlic and cook until fragrant, about 30 seconds. Stir in tomatoes and sugar; increase heat to high and bring to simmer. Lower heat to medium-low and simmer until thickened slightly, about 10 minutes. Off heat, stir in basil and oil; season with salt and pepper. Serve.