Heat olive oil in a large skillet over medium-high heat. Season chicken with salt and pepper, then cook for 5-6 minutes until browned and cooked through. Remove to a plate.
Add onion to the same skillet and cook for 3-4 minutes until softened. Add garlic and cook for 1 minute until fragrant.
Stir in orzo, chicken broth, sun-dried tomatoes, and oregano. Bring to a boil, then reduce heat to medium-low, cover, and simmer for 12-15 minutes until orzo is tender and liquid is absorbed, stirring occasionally.
Return chicken to the skillet along with spinach and olives. Stir until spinach wilts, about 2 minutes.
Remove from heat and stir in feta cheese. Taste and adjust seasoning with salt and pepper before serving.