Heat olive oil in sauce pan.
Cook onion and garlic in oil until soft, about 5 minutes.
Add tomato sauce, chili powder, oregano and salt.
Simmer while you prep other ingredients.
Combine chicken and all but 1/2 cup of cheese.
Fill tortillas with chicken and cheese.
Spray 9x13 casserole dish with non-stick cooking spray.
Spread small amount of sauce on bottom of dish.
Place filled tortillas in dish.
Pour sauce over the top of them.
Sprinkle with remaining 1/2 cup cheese.
Bake at 350 degrees for 20 minutes.