Preheat oven to 325º. Heat a large stockpot (8 quart) over high heat for a minute, then add oil, salt and popcorn.
Cover and cook for a few minutes, shaking the pot vigorously until the popping has mostly finished. Remove immediately from heat; pour popcorn into two large bowls. Divide nuts, if using, between the two bowls.
In a small saucepan, heat the butter, syrup and a few good pinches of sea salt over medium heat. After small bubbles start to form, cook 4-5 minutes, whisking occasionally until thick and foamy.
Remove from heat, pour over popcorn in bowls and quickly mix together with two spoons.
Divide popcorn between two baking sheets and bake for 10 minutes, stirring halfway through.
The maple caramel corn will crisp up as it cools. Store in a tightly sealed container after it’s cooled completely to preserve the crispness.