After sugar reaches the hard ball stage, it requires some speed in handling so the candy doesn't set. You may want to halve the recipes using cooked sugar the first few times you make them. If you make these balls to use as tree ornaments, be sure to pop some extra corn for munching.
In a large bowl mix the popped popcorn with the toasted oat cereal and set aside. In a heavy, medium-sized saucepan mix the sugar, syrup, water and vinegar. Cook over medium heat, stirring constantly, until mixture reaches 260 degrees on a candy thermometer, or until a bit of the mixture dropped into iced water forms into a hard ball.
Pour syrup over popcorn. Stir gently with an oiled wooden spoon until the popcorn is well coated. Allow to cool slightly, and when it is cool enough to handle, with buttered fingers press into balls. To use as ornaments, wrap them in red and green ribbons. Makes around 18 three-inch balls.