Grease a 9×13 inch baking dish. In a large bowl, microwave margarine for 45 seconds or until melted. Add marshmallows and stir to coat. Return to the microwave for 1 1/2 more minutes, stirring after 45 seconds. Mix in the peanut butter, then immediately stir in the toasted oat cereal. At this time you can add any additional candies. Transfer the mixture to the prepared pan and press down with a greased spatula until even. Allow to cool before cutting into squares.