Heat oven to 425°F. Toss eggplant with 2 tablespoons oil and its salt on rimmed sheet. Roast 22 to 25 minutes, turning once, until tender with browned edges.
Meanwhile, warm remaining oil in saucepan; stir garlic 30 seconds. Add tomatoes, water, oregano, and pepper; simmer 10 minutes.
Boil penne with tablespoon salt to 3 minutes short of package time; drain. Reduce oven to 375°F.
Fold penne, eggplant, olives, and mozzarella into sauce. Transfer to deep 9-by-13-inch dish; add Parmesan.
Foil-cover; bake 20 minutes, then uncovered 8 to 10 minutes until center 165°F and surface lightly browned. Rest 10 minutes.