Heat olive oil in a large skillet over medium-high heat. Add onion; sauté 15 minutes or until golden. Place potatoes in a saucepan; cover with water, and bring to a boil. Cover, reduce heat, and simmer 15 minutes or until tender; drain. Stir in buttermilk, butter, salt, and black pepper; mash with a potato masher. Stir in onion.