Preheat oven to 350 degrees F. For sauce, melt butter over medium heat in a small saucepan. Stir in flour, garlic salt, dried oregano (if using) and pepper. Stir in milk. Cook and stir over medium heat until thick and bubbly. Cook and stir for 1 minute more. Remove from heat. Stir in sour cream and fresh oregano (if using). Place beans in a 1-1/2-quart baking dish. Pour sauce over and toss to coat.
Toss pita strips and oil in a medium bowl. Top casserole with pita strips. Bake, uncovered, for 30 minutes or until pita strips begin to brown and green bean mixture is heated through.