In a mixing bowl, combine chopped onion, can of chopped olives and oregano.
Heat oil in a frying pan. Fry corn tortillas, one at a time, in hot oil just until softened, about 10-15 seconds per side. Set aside.
Pour a scant ⅓ cup of sauce in a 13X9 inch casserole dish. Tilt to coat the bottom of the dish.
Dip each tortilla in the enchilada sauce to coat and lay it in the casserole dish.
Place about a tablespoon of chopped olive/onion/oregano and about a ¼ cup of cheddar cheese in each tortilla. Roll up each tortilla and place seam side down.
Continue this process using the remaining tortillas, chopped olive/onion/oregano filling and cheese.
When all the tortillas are filled and rolled in the dish, top with remaining sauce and cheese.
Top enchiladas with sliced olives.
Bake at 350° for 25 to 30 minutes or until enchiladas are heated through and cheese is hot and bubbly.
Allow casserole to rest for 10 minutes before serving to allow filling to set.
Serve enchiladas topped with a dollup of sour cream and chopped cilantro.