Preheat your oven to 425°F (220°C).
In a large bowl, combine olive oil, garlic powder, onion powder, smoked paprika, dried oregano, salt, black pepper, and red pepper flakes (if using). Mix well to create a marinade.
Add the chicken thighs to the bowl and toss until they are evenly coated with the marinade.
Arrange the chicken thighs skin-side up on a baking sheet lined with parchment paper. Place lemon slices around the chicken.
Bake in the preheated oven for 35-40 minutes, or until the chicken reaches an internal temperature of 165°F (75°C) and the skin is crispy.
Remove from the oven and let the chicken rest for 5 minutes before serving. Garnish with fresh parsley if desired.