Preheat the oven to 425°F (220°C).
Pat the chicken thighs dry and season both sides with the dried oregano, salt, and black pepper.
Heat 1 to 2 tablespoons olive oil in a large oven-safe skillet over medium-high heat.
Sear the chicken thighs skin-side down for about 3 minutes until golden, then flip and sear the other side for 3 minutes.
Transfer the skillet to the preheated oven and roast the chicken for 10 to 15 minutes, or until cooked through. Remove chicken to a plate and keep warm; pour off and discard excess fat from the skillet.
Return the skillet to the stovetop over medium heat, add the minced garlic and sauté briefly until fragrant (about 30 seconds to 1 minute).
Add the uncooked orzo, chicken broth, and halved Kalamata olives to the skillet. Bring to a boil, then reduce heat to low, cover, and simmer for about 15 minutes or until the orzo is tender and most liquid is absorbed.
Stir in the fresh dill, lemon zest, and baby spinach until the spinach wilts slightly.
Return the cooked chicken thighs to the skillet to warm through. Sprinkle the feta cheese, chopped roasted red peppers, and diced purple onion over the top before serving.