Line a baking sheet with parchment paper or wax paper, or place a wire rack over a piece of parchment, and set aside.
Add Halloween sprinkles to small bowls for easy access. Set aside.
Place the purple, green, and orange candy melts into separate small microwave-safe bowls.
Beginning with one bowl, microwave the candy melts for a 30-second interval, then stir. Continue microwave in 15-second intervals, stirring after each, until the candy melts are completely melted and smooth. Repeat with the remaining two bowls.
Using a fork or dipping tool, lower an Oreo cookie into one color of melted candy until fully coated. Allow the excess to drip off before placing the cookie onto the prepared baking sheet or wire rack.
Continue dipping, so you have a total of 4 Oreo cookies in purple, 4 in green, and 4 in orange candy melts.
While the candy coating is still wet, generously sprinkle the Halloween sprinkles over the top of each Oreo, or gently press candy eyeballs into the coating.
Allow the decorated Oreos to cool at room temperature until the candy coating has completely hardened. This can take about 30 minutes to an hour, depending on the thickness of the coating and the temperature of your house. If you're in a hurry, you can place the baking sheet in the refrigerator for about 15 minutes to speed up the process.
Once the candy coating has set, serve and enjoy!