Heat oil and a few popcorn kernels in a large stockpot, covered, over high heat. When they pop, add remaining popcorn kernels. Cook, shaking pot, until popped. Transfer popcorn to a large bowl.

Preheat oven to 250 degrees. Heat butter, sugar, and corn syrup in a small saucepan over high heat. Cook, stirring, until butter and sugar melt and a candy thermometer reaches 255 degrees. Remove from heat, and stir in rosemary, salt, and baking soda. Pour over popcorn; toss. Transfer to a baking sheet.

Bake, stirring occasionally, until dried, about 45 minutes. Let cool before serving. Popcorn can be stored in an airtight container at room temperature for up to 3 days.