Cut a 3" slit in the thick side of each chicken breast to form a pocket, being careful to not cut completely through the flesh. Combine cheese, parsley, oregano and oil and mix gently. Spoon 1/4 of filling into each pocket. Place chicken in ungreased 2 quart baking dish.
In another small bowl, mix tomatoes, olives and cornstarch and pour over chicken. Bake at 350 degrees for 35-40 minutes until chicken is thoroughly cooked. Serve with spaghetti, if you like.