In a medium bowl, stir together the flour, garlic salt, pepper, and oregano. Dredge chicken in the mixture to lightly coat.
Heat olive oil and butter in a large skillet over medium heat. Fry the chicken in the skillet for 2 minutes, or until lightly browned on one side. Turn chicken over, and add mushrooms and minced garlic. Cook about 2 minutes, until other side of chicken is lightly browned. Stir mushrooms so that they cook evenly.
Pour Marsala wine and mushroom soup over the chicken. Cover skillet, and reduce heat to low; simmer for 10 minutes, or until chicken is no longer pink and juices run clear.