Mix garlic powder, onion powder, smoked paprika, chili powder, dried oregano, salt, black pepper, and cayenne pepper (if using) in a bowl.
Pat the chicken thighs dry with paper towels and season both sides with the spice mixture.
Line a baking sheet with parchment paper and brush with olive oil. Arrange the chicken thighs in a single layer.
Bake for 20 minutes, then sprinkle with brown sugar (if using) and bake for an additional 5 minutes.
Let the chicken rest for 5 minutes, garnish with chopped parsley, and serve.
Notes
You can adjust the amount of cayenne pepper or omit it for a milder flavor. The brown sugar adds a nice sweetness but can be skipped for a savory version.