In a large saucepan, stir together the oil and popcorn kernels. Cover and cook over medium heat just until you hear the kernels begin to pop, 3 to 5 minutes. Tilt the lid slightly, allowing steam to escape. Cook until the popping slows, about 2 minutes. Remove from the heat and let stand until the popping stops, about 30 seconds.
In a large bowl, mix the cocoa powder, sugar, cinnamon and salt. Sprinkle over warm popcorn; mix to coat. Sprinkle with chocolate chips and marshmallows and mix again.