Preheat oven to 350°.
Place the potato and onion in a roasting pan coated with cooking spray. Drizzle with 1 tablespoon plus 2 teaspoons oil, and sprinkle with 1/2 teaspoon salt. Toss well. Bake at 350° for 1 hour or until potatoes are tender, stirring occasionally with a metal spatula to prevent sticking.
Combine eggs and 1/4 teaspoon salt in a large bowl. Stir in potato mixture; let stand 10 minutes. Heat 1 teaspoon oil in an 8-inch nonstick skillet over medium heat. Pour potato mixture into pan (pan will be very full). Cook 7 minutes or until almost set, gently shaking pan frequently.
Place a plate upside down on top of omelet; invert onto plate. Carefully slide omelet cooked side up into pan; cook 3 minutes or until set, gently shaking pan occasionally. Carefully loosen omelet with a spatula; gently slide omelet onto a plate. Cool. Cut into wedges. Garnish with oregano, if desired.