Put oven rack in upper third of oven and preheat oven to 475°F.
Cut parsnips lengthwise into quarters, then cut out and discard any tough core. Cut parsnips diagonally into 2-inch pieces. Cut squash crosswise into 1/2-inch-thick slices, then cut slices into wedges. Toss vegetables with salt, pepper, and 1 1/2 tablespoons oil, then spread in 1 layer in a large shallow baking pan. Roast, turning occasionally, until vegetables are tender and browned, 20 to 25 minutes.
While vegetables are roasting, heat remaining 1 1/2 tablespoons oil with garlic in a 1-quart saucepan over low heat, stirring occasionally, until garlic just begins to sizzle. Remove from heat.
When vegetables are done roasting, remove garlic from oil (discard garlic) and toss vegetables with garlic oil, olives, and parsley.
