Preheat oven to 425 degrees. Cut a thin (1/4-inch) slice from the top of each head of garlic; discard. Place garlic in a small baking dish. Drizzle with oil. Cover with parchment paper-lined foil; roast garlic until soft and golden, about 1 hour and 15 minutes. To store, squeeze out unused cloves into jar and add enough olive oil to cover. Store closed jar in the refrigerator for up to one week.