Heat the oil in a medium skillet over medium heat. Add the onion and oregano. Cook, stirring until the onion is barely tender, 2 to 3 minutes. Transfer to a bowl. Stir in the vinegar and sugar. Season with salt and pepper. Marinate for at least 1 hour or overnight. Serve the onions at room temperature, or sizzle the onions briefly in a hot skillet just before serving. *Serve as a condiment with all meats, poultry, fish, on sandwiches or as part of a salad. Makes 1 cup