Prepare the Apples: Wash apples thoroughly and dry well. Insert a wooden stick firmly into the stem end of each apple. Line a baking sheet with parchment paper and lightly grease it.
Cook the Syrup: In a heavy saucepan, combine sugar, corn syrup, and water. Stir gently over medium heat until the sugar dissolves, then bring to a boil without stirring.
Flavor and Color: Continue boiling until the syrup reaches hard crack stage (300°F / 150°C) on a candy thermometer, about 10–12 minutes. Remove from heat, then quickly stir in the cinnamon oil and red food coloring.
Coat the Apples: Working quickly, tilt the pan and dip each apple into the syrup, twirling to coat evenly. Let excess drip off, then place apples on the prepared baking sheet to cool and harden.
Serve: Allow to cool completely until the candy shell is firm. Candied apples are best eaten the same day for maximum crunch.