Prepare the marinade. Mix the mustard powder, garlic powder, onion powder, sweet paprika, 1 teaspoon dried oregano, olive oil, and ½ teaspoon sea salt in a small bowl to form a paste.
Marinate. Rub the marinade all over the chicken under the skin (pull with your hands and rub the marinade inside). Marinate for 2-3 hours or even better overnight.
Preheat oven to 400°F / 200°C. Meanwhile, remove the chicken from the fridge and allow it to sit at room temperature for 20 minutes while you preheat the oven. Pat dry the chicken with paper towels.
Brush a baking pan or tray with olive oil. Add the chicken on top. Alternatively, you can place the chicken on a rack (See Notes below).
Brush or rub the skin of the chicken lightly with olive oil. Sprinkle the remaining ½ teaspoon of sea salt on the skin as well as the remaining 1 teaspoon of oregano.
Bake for 35 to 45 minutes on the oven's middle rack. Or until the chicken reaches an internal temperature of 165°F / 74°C and the skin is nice and crispy.
Rest the chicken for 5 minutes out of the pan and then serve with your preferred side dishes.