In a heavy-bottomed pot with a tight-fitting lid, stir together oil, sugar, and popcorn until kernels are coated.
Cover pot and place over medium-high heat.
Heat for a minute or so, shaking once or twice for good measure, until you hear kernels begin to pop.
Wearing oven mitts, grasp pot handles, using your thumbs to hold the lid on tightly, and lift pot just off the burner.
Shake, hard, from side to side until popping slows down to about 1 second between pops.
Remove from heat.
Sprinkle with cinnamon and salt and toss well.
Transfer to a large serving bowl and cool fully.
