Pierce potatoes with a fork; arrange in a circle on paper towels in microwave oven; cover with wax paper. Microwave at high 5 minutes or until crisp-tender; cool. Thinly slice potatoes.
Lightly spoon flour into a dry measuring cup; level with a knife. Combine flour and broth in a medium saucepan; stir with a whisk. Stir in milk and next 5 ingredients (milk through garlic). Bring mixture to a boil over medium heat, and cook until thick (about 10 minutes), stirring constantly with a whisk. Remove from heat; add cheese, stirring until cheese melts.
Arrange half of potato slices in the bottom of a 13 x 9-inch baking dish coated with cooking spray. Top with half of the onion, oregano, zucchini, yellow squash, and cheese sauce. Repeat procedure with remaining potato, onion, oregano, zucchini, squash, and cheese sauce.
Bake at 375° for 1 hour or until gratin is tender. Combine breadcrumbs and oil in a small bowl; toss well. Sprinkle breadcrumb mixture over gratin.
Broil gratin 30 seconds or until breadcrumbs are lightly browned. Let stand 10 minutes.