Heat the oil in a large saucepan over medium heat. Add the onion, ½ teaspoon salt, and ¼ teaspoon pepper. Cook, stirring occasionally, until soft, 3 to 5 minutes.
Add the sweet potatoes and garlic and cook, stirring occasionally, for 1 minute.
Add the rice and cook, stirring, for 2 minutes. Add the wine and cook, stirring frequently, until absorbed.
Measure 3½ cups of water. Add ¾ cup at a time and cook, stirring occasionally and waiting until each addition is absorbed before adding the next. It should take 25 to 30 minutes for all the water to be absorbed.
Stir in the Parmesan and oregano.