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. Cook about 2 minutes, until other side of chicken is lightly browned. Stir mushrooms so that they cook evenly.
Pour Marsala wine 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.