In a medium bowl stir together the flour, garlic salt, pepper and oregano. Coat chicken in the mixture to lightly coat.
Heat olive oil and butter in a large frypan over medium heat. Fry the chicken in the frypan 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 frypan and reduce heat to low; simmer for 10 minutes or until chicken is no longer pink and juices run clear.