Preheat oven to 425°.
Remove papery husks from garlic. Place garlic on a square of aluminum foil; drizzle with oil, and wrap in foil. Bake at 425° for 30 minutes; set aside.
Place potato in a medium saucepan; add water to cover. Bring to a boil; reduce heat. Cook, uncovered, 15 to 20 minutes or until tender; drain. Mash potato with a potato masher. Add milk and remaining 3 ingredients; mash until fluffy.
Cut off ends of garlic; squeeze pulp from cloves into potato mixture, and stir.