Line a large rimmed baking sheet with parchment paper or a silicone baking mat. Make sure the sheet is large enough to spread the popcorn mixture into a fairly even layer.
Place the popped popcorn in a very large mixing bowl. Remove any unpopped kernels so they don’t end up in the finished snack. If using homemade popcorn, let it cool completely before adding the other ingredients.
Coarsely chop the Halloween Oreos into bite-sized pieces. Add them to the bowl with the popcorn and Halloween M&M’s. Toss gently to distribute the mix-ins without crushing the popcorn.
Combine the white chocolate chips and vegetable oil in a microwave-safe bowl. Microwave in 20–30-second intervals, stirring well after each interval, until the chocolate is smooth. Avoid overheating, as white chocolate can scorch easily.
Pour the melted white chocolate over the popcorn mixture. Use a silicone spatula to gently fold everything together until the popcorn, cookies, and candy are lightly coated. Work quickly because the chocolate begins to set as it cools.
Transfer the mixture to the prepared baking sheet and spread it into an even layer. Immediately sprinkle Halloween sprinkles over the top, if using, while the chocolate is still wet.
Allow the popcorn to sit at room temperature for about 30 minutes, or refrigerate briefly until the chocolate hardens. Break the mixture into bite-sized clusters and transfer to a serving bowl or treat bags.
