Preheat oven to 250 degrees F.
In a large dutch oven, add the oil. Heat over medium heat until melted. Add 3-5 kernels of popcorn. Cover and wait until all kernels have popped, 1-2 minutes. Remove the popcorn with a slotted spoon and throw away. Reduce the heat to medium low, and add the rest of the popcorn kernels. Sprinkle with salt. Cover and allow the popcorn to pop until there is a couple of seconds between each pop, about 2-3 minutes. Transfer the popcorn to a large roasting pan. Set aside.
In a heavy bottomed saucepan, combine the brown sugar, corn syrup, butter, and salt. Bring the mixture to a boil over medium heat, stirring constantly. Stir in the baking soda and continue to cook until the mixture is thick. Remove from heat and pour the caramel over the popcorn. Toss to coat evenly.
Bake the popcorn for 1 hour and 10 minutes or until crispy. Toss the popcorn every 15 minutes while it bakes. Remove from heat and pour the popcorn onto a couple of baking sheets that have been lined with foil. Break up the popcorn once it's cool enough to handle.
Once the caramel corn has cooled completely transfer it to airtight container where it will keep for a week or two.