Preheat your oven to 400°F.
In a large bowl, combine the vegetable oil, fish sauce, soy sauce, brown sugar, minced garlic, grated ginger, black pepper, and red pepper flakes (if using). Mix well to create a marinade.
Add the chicken thighs to the marinade, ensuring they are well coated. Cover and refrigerate for at least 30 minutes, or up to 2 hours for more flavor.
Place the marinated chicken thighs on a baking sheet lined with parchment paper, skin side up. Reserve any remaining marinade.
Bake in the preheated oven for 35-40 minutes, basting with the reserved marinade halfway through, until the chicken is cooked through and the skin is crispy.
Remove from the oven and let it rest for 5 minutes. Garnish with chopped green onions and fresh cilantro before serving.