Coat a rimmed baking sheet with oil. Combine sugar, water, and corn syrup in a large saucepan over medium-high; bring to a boil, stirring occasionally. Cook, without stirring, until a candy thermometer inserted in mixture registers 264°F, about 20 minutes. Stir in cashews and salt. Cook, stirring occasionally, until mixture reaches 318°F, about 15 minutes. Remove pan from heat. Stir in butter, orange zest, rosemary, baking soda, and vanilla. Immediately pour mixture into prepared pan, tilting pan to level candy. (Candy will not reach edges of pan.) Let stand until hard, 1 to 1 1/2 hours. Break brittle into pieces. Store in an airtight container for up to 2 weeks.